Web Browsers are built on multithreaded rendering - Paint, Layout and Main threads. It works really well, but on legacy or native everything is done on the single main thread and everything is jammed into 16ms of a single thread.

For example why iOS is so fast? It renders everything in separate process. (This is why Flutter will always be a garbage by reimplementing this in main thread).

I bet it just too much and too old to rewrite to new shiny things.