That only works if you don't care about fragmenting what features you support on multiple OSes, and trust those codes to do the right thing in all of your use c

That only works if you don't care about fragmenting what features you support on multiple OSes, and trust those codes to do the right thing in all of your use cases for it. For some of the browsers it's clearly better for them to bundle the kitchen sink, since at least it's their kitchen sink and guarantees the kitchen sink will always be there.