Many big companies (especially vendors) do contribute with people and otherwise--because, as you say, it does often help themselves--and most significant developers of major open source projects are doing it as part of their day job.

There's a long history of licenses in the PC freeware/shareware space of being only for personal use/educational use/etc. That usage-based restrictions did not make it into the FOSS world is an important reason that it's been so successful IMO.