Writing performant, clean, pure software is super appealing as a developer, so why don't I do something about the bloated software I write? I think a big part of it is it's hard to see the direct benefit from the very large amount of effort I'll have to put in.

Sure I can write that one thing from that one library that I use myself instead of pulling in the whole library. I might be faster, I might end up with a smaller binary, it might be more deterministic because I know exactly what it's doing. But it'll take a long time, might have a lot of bugs and forget about maintaining it. Then end of the day, do the people that use my software care that I put in the effort to do this? They probably won't even notice.