Why? That's certainly not what happened with Google's browser.
Because they can outspend you in engineering resources to push forward, and working with someone elses stack is a burden, not a blessing, if the other side moves more quickly (and forces you to rebase).
I don't remember arguing that developing a modern browser is cheap - I'm responding to the strange notion that the availability of a full open source browser from Google somehow makes things harder for competitors. It doesn't. And, again, as to forking - Apple created a competitive browser by forking an opensource project, Google created another one forking Apple's. This is demonstrably a viable and arguably more effective way of developing a browser.
Why do you think I don't see that?
On top of that, if you have to compete in the browser space by forking off of Google's rendering engine and web platform, you're always going to be behind.
Why? That's certainly not what happened with Google's browser.