Because Windows doesn't go open-source and others can't build their OS from windows like chromium. With OS, there are no open source kernels that are actively maintained and security-fix bump every month by full time staff of giant corporation. With browsers, devs already have an open source engine with most of the work and build are from full-time staff of a giant corporation, and then they just lazily build "their own" browsers upon that and brag on social media.

Build your own browser engine and see how you can pay the devs to make them work on it.