If this happened then JavaScript would be split into 20+ languages, one for each popular compiler. There would be nothing stopping a tool maker from adding their favorite language features even if no other compiler ever adopted them. That would be a disaster.
Outside of browsers this is already how it works. Nothing is stopping LLVM versus GCC from adding their favorite language features, but it's not a disaster.
It is a disaster the moment you try to run same code on different platforms.