Many core Python contributors including Guido are employed by companies like Google to work on it.
Mozilla is, technically, an exception: almost everyone is employed by the Mozilla corporation, a non-profit by virtue of its lucrative search deal with Google. Few projects have that luxury.
1) To work full time on something like Linux, it really requires an external employer. So it isn't surprising that this is the case. To get involved does not, and certainly for a long time independent developers were as important as corporations there.
2) Everyone economically owns the output. I don't mean the copyrights. I mean everyone has a right to put the software to use for whatever purposes they want, whether in manufacturing, say, server appliances, or just running a web site (or even on my wife's laptop).
If the big companies stopped contributing to Linux tomorrow, would Linux die? I doubt it. So I suppose it depends on what you mean by "dependent." Certainly Linux is not dependent on corporate coding the way, say, Windows is.....