Who is "we" and how could "we" stop them exactly?

"We" is Browser makers and users. We stop them by taking back control of what runs on our computer.

Because when implemented properly it is a better user experience.

if done properly, this allows the url to represent where you are in the app, and makes back/forward work - like in typical web pages, just without a reload.

Most often though, I see it done inconsistently, and it's a painful mess

Because some sites need to change those things. So they can't just deny it for everybody.