Because it’s not a primarily solution to transferring money, it’s primarily decentralized, trustless and censorship-resistant solution to that. Of course centralized solutions are cheaper and more efficient.
not at all like central banks because mining is decentralized. Besides, there are also users and merchants who are just as important and miners don’t dare pushing changes that nobody wants.
Do you realize that of miners decided to fork in favor of a model which is better fit them, there will be a shortage of block mining until the difficulty declines. Which means transactions would be frozen for around a month. I'm not sure users and merchands could stand that fight …
Rational miners realize that forcing a protocol change that isn’t acceptable to users or merchants will destroy valuation of the fork they choose to mine, so there is discebtive from such behavior.
But yes, if they do it anyway, there will be period with very slow blocks and that will hurt the project a lot but then difficulty will adjust downward.
