Because decentralization takes more energy than centralized systems.
Thats not always true, have a look at Gnutella.
You have to waste more and more energy/power/electricity day-to-day and month-to-month just to secure transactions
No you don't, we only need enough hashpower to mitigate a 51% attack, it doesn't need to keep increasing, the only reason the hashrate is increasing right now is because mining equipment has gotten much more efficient in a very short period of time and more and more people are entering the bitcoin mining industry.