Browsers are already bloated as it is, why not just add all those "popular" libraries to the browser and make things local fetch by default? No, i'm not talkin

Browsers are already bloated as it is, why not just add all those "popular" libraries to the browser and make things local fetch by default? No, i'm not talking browser extensions because that would require users to proactively install something for a webmaster (yeah, i said "webmaster", i don't care if it shows my age)...I'm talking about integrating the top X most implemented javascript libraries into the browser itself...why not just do that i wonder? I'm half-winking as i state this (bloated aspect) because i give much respect to browser makers for all that browsers actually do/have to do for us when rendering web experiences. So, i don't take adding even more stuff into browsers as a simply, hand-wavy thing to consider. But I also wonder if these libraries do in fact provide such helpful (and ubiquitous) utilities, why not incorporate them into the default - what is called - ecmascript?