There are many projects that are managed by foundations, like the Apache Foundation, FSF, Linux Foundation, etc. Those projects aren't backed by any one corporate entity.

Aside from that, there's still value in contributing to a product you're consuming at your day job: not having to maintain forks. If you can get your feature into the upstream project, less work for you in the long run, it's a win win.

Should you contribute to corporate owned projects in your free time for the fun of it? Probably not, unless you think it will earn you some kind of recognition (resume fodder).