Is the frontend treadmill responsible for making browsers so expensive to maintain? Sorry for the incredibly naive question. I've only ever worked on "backend"

Is the frontend treadmill responsible for making browsers so expensive to maintain? Sorry for the incredibly naive question. I've only ever worked on "backend" projects. A few times I've made simple read-only pages in raw html (and maybe some CSS) to share information with colleagues. So while I get that the web is more awesome now than it was 10 years ago, I don't have any concrete understanding as to why this is a multi-billion dollar industry.