The challenge with browsers and extensions is the same problem with pretty much all software: economics. Software isn't free, and developing it costs time and often money.
Browsers themselves have a money spigot: Search engine revenue ($110M USD per % of marketshare, if Firefox's numbers are any guide).
Most extensions are only money-adjacent-- They have the technical ability to do things that hijack revenue (redirect searches, show their own ads, camp on affiliate codes, etc). But they're generally not supposed to do these things.
Users won't (in large), pay for them directly.