I don't like the "bring your own" approach because unless one happens to be an absolute tour de force 10x engineer unstoppable god of a programmer (which most of us, myself included, are not), whatever you build is never going to be as well-rounded, fleshed out, and complete as something built by a larger organization, especially when it comes to UI libraries (which are monstrous projects if done right, e.g. meeting accessibility requirements).

I'm happy to contribute to a larger effort but anything I can build on my own is going to be a thin, flimsy happy meal toy compared to something with the backing of a company or well organized FOSS project.