Like who for instance? Basically only Blink, WebKit and Gecko are usable non-toy web engines these days, and they're all backed by big companies with deep pockets and many engineers.

Opera's long gone, now it's a chromium fork. Internet Explorer is gone, now MS uses a chromium fork. The hype new browsers like Brave and Vivaldi are just chromium under the hood.

It's like the difference between making a Linux distro and maintaining a full OS.