Why on gods green earth, would anybody look at a file with 20k lines, which clearly was made by something called WEBPACK and decide it's the right place to implement their changes for the next few years?!?

Because they can.

"It's easier to do so, I only have to search and debug only one file"

cause it’s easier to change something, forget about it and go drink some beer instead of wasting time to setup environment in no longer maintained code, you just need to get older and start being lazy like everyone who finish their 5 days sprint on Wednesday