Why is Bitcoin transaction not reversible

[pooya87]

Bitcoin was invented to solve problems that existed in the existing payment systems. One of the issues Bitcoin addresses is reversibility of transactions. Take the traditional payment systems: banks, credit cards, PayPal, ... one of the issues the "merchants" have faced for years (specially with things like PayPal) has been getting ripped off by their customers. Just search how many complaints have existed over the years by PayPal users (receivers/merchants) who got their transactions reversed.

Bitcoin was created to give merchants that reliable option to be able to receive a payment and be sure it won't be reversed.

However, once a transaction is broadcast and confirmed on the Bitcoin network, they cannot reverse it.

It's more accurate to say when the transaction is confirmed (is included in a block) then it becomes very difficult to reverse it since it requires a massive computing power to re-produce the same block and build on top of it to get the longest chain.
After that with each confirmation (as the chain grows and that tx goes deeper) it becomes exponentially more difficult to reverse it.

[bitbollo]

...
This is why most wait five or more confirmation
Because it gets more expensive as the confirmation increases for a reorg.

No way...You can easily trust bitcoin transaction after 1 confirmation!
I would never expect to wait 5 confirmations like early times for confirming a transaction with some one... no valid reason at all, once confirmed is confirmed.
This is not only me but also any kind of service has not this kind of request. Would you expect that some one would try to make a 51% attack for stealing a dozen of bucks and reverse a payment?
Definetely not possible... this was an issue of the past (like early times on 2013-2014) suggested for avoid forks - not use the longest chain...

[Ambatman]

...
This is why most wait five or more confirmation
Because it gets more expensive as the confirmation increases for a reorg.

No way...You can easily trust bitcoin transaction after 1 confirmation!

Rare but Natural reorganization occurs if a competing chain becomes the accepted chain.
The transaction In the old block would leave the blockchain.
Let's say
The latest Block is block 10
Miner A and Miner B both found Block 11A and Block 11B respectively at almost similar time
Different part of the network receives different version first
During that period some node would think Block 11A while others Block11B
Now if another or Block 12 is found on Block 11B
It becomes the longest chain or better still one with the most work.
While Block 11A becomes an orphan block

I would never expect to wait 5 confirmations like early times for confirming a transaction with some one... no valid reason at all, once confirmed is confirmed.
This is not only me but also any kind of service has not this kind of request.

Excahnges and some other platform wait for at least three confirmation.

Would you expect that some one would try to make a 51% attack for stealing a dozen of bucks and reverse a payment?

51% attack isn't the only way Reorg occurs

[SilverCryptoBullet]

Bitcoin was invented to solve problems that existed in the existing payment systems. One of the issues Bitcoin addresses is reversibility of transactions. Take the traditional payment systems: banks, credit cards, PayPal, ... one of the issues the "merchants" have faced for years (specially with things like PayPal) has been getting ripped off by their customers. Just search how many complaints have existed over the years by PayPal users (receivers/merchants) who got their transactions reversed.

Bitcoin was created to give merchants that reliable option to be able to receive a payment and be sure it won't be reversed.

They only need to wait for 1 confirmation to avoid double spend and if want more certainty they can wait for 2 confirmations for avoiding reorg. After that, transactions can be considered as laid solidly on Bitcoin blockchain and the trade between two people, customer and a merchant can be finalized for most transactions.

Except can be for transactions with big values, then 6 confirmations can be a higher standard.

https://jlopp.github.io/bitcoin-confirmation-risk-calculator/
The standard for high value payments is to wait for 6 confirmations, but it's not that simple!

Would you expect that some one would try to make a 51% attack for stealing a dozen of bucks and reverse a payment?

51% attack isn't the only way Reorg occurs

it seems 51% attacks and reorgs are different (likely in purposes). Years ago there was a reorg event on Bitcoin blockchain and Andreas Antonopoulos had a livestream about that.

Likely not this video, I remember it was a shorter one.
Cryptocurrency Explained: Understanding "Double-spend", Block Re-Organization, & Consensus [Bitcoin]

[Franklyn-wood]

...
This is why most wait five or more confirmation
Because it gets more expensive as the confirmation increases for a reorg.

No way...You can easily trust bitcoin transaction after 1 confirmation!
I would never expect to wait 5 confirmations like early times for confirming a transaction with some one... no valid reason at all, once confirmed is confirmed.
This is not only me but also any kind of service has not this kind of request. Would you expect that some one would try to make a 51% attack for stealing a dozen of bucks and reverse a payment?
Definetely not possible... this was an issue of the past (like early times on 2013-2014) suggested for avoid forks - not use the longest chain...

The higher the transaction the higher the security needed to mine new cryptocurrency transactions successfully just like our local currency. The processing time is always different due to the amount processed, it takes about 8 to 10 minutes per block, it's not only about the confirmation but also the time used in processing it matters.
There is no way Bitcoin can be reversed when you send it, because not middle man or agents to monitor it obeys commands giving to it.

[Hardyrobust]

What you said is correct OP. Anyone not been able to reverse transaction after they have being initiated is of great advantage to bitcoin. With this feature it is impossible to scam someone, the sender of the bitcoin and the receiver is being protected.
While initiating a transaction, we need to be careful by checking the network and wallet address if it is correct. Any form of mistake is not reversible once transaction has been initiated and this will result to lost of money, the funny part is there is no customer care that will be contacted if there is a mistake.
It is always advisable for newbies not to start with initiating a transaction that involves big amounts rather they can start with a smaller amount as a kind of learning. While doing this , the transaction fee may be high but it is far better than losing ones entire bitcoin to the wrong wallet address.