My suspicion is it boils down to maturity and scope of the domain. When it is brand-new, one guy can change the world in his garage in a few months. As it matures, you start to need large teams to build the next incredible thing. Even if you are passionate and ambitious, tinkering in your garage simply will not allow you to realize your vision. So you join a large company where you can.

Random datapoint, everyone I knew who was deeply passionate about compilers seemed to be going to work for Apple for a while. They could have written a new research compiler in their basement - but Apple is where LLVM was happening.