I also know browsers have continuously gotten better at dealing with and adjusting to bad frontend performance
Citation needed
But I would counter that frontend performance is as important now as ever
Citation needed - it certainly feels like frontend performance was rarely seen as important in the past 10+ years.
So, again, anyone have a good ideas or thoughts as to why this field has erupted, and seemingly absorbed frontend?
Probably for the same reason that "programmers" got separated into "backend/frontend", so that it feels like you have more specialists, instead of asking them to do a good job in the first place.