I can't speak to the complexity you've encountered, but for me it's pretty much zero. A button component is just a function. React-Router is good enough and code splitting is pretty much just changing how to import something. Component state is dead-easy to write by just adding a useState hook. Bundlers pretty much handle everything these days so not to much concern about size.
Your view on front-end developers having been mediocre in the past isn't far off though, at least in my experience. I noticed a big difference between the people who wanted to build nice looking pages and the ones that wanted to build applications myself. Even today it amazes me how many people have never unit tested their code, have no idea about layering an application and have poor JS/TS fundamentals. It's gotten a lot better though.
Ultimately it isn't perfect for everything, but for a lot of people it's an easy choice. And for me personally, the tons of other JS frameworks do very little in that area that I'd pick them. I'd rather spend my time working on the product. Lol, maybe its just the default because its the default at this point.