If the blockchain cannot be hacked, how can you lose anything?

[Imnon]

all hack tries against sites and wallet not transaction.
A change can occur in some things, such as an attack of 51%, but it is considered a violation.
Once you send the transaction and it is confirmed, you are safe.
If you keep your private key in a safe place, you will be fine.
if you have hardware wallet and safe PC no one can hack your bitcoins.