Browser vendors have spent the past several decades trying to fix those weaknesses and haven't, it seems like a very strong indication that it is not exactly an easy thing to do.
This might be just because most web apps never had a bottleneck on JS->DOM changes.
This is blatantly false, as the DOM has always been a bottleneck for web apps.
Because you can't "just" fix them. It's a complex problem with no easy solutions. Don't you think it'd already be "fixed" if it was easy?