When I look at the best programmers I know, none have them have done any open source work whatsoever. This group of people ranges from IBM fellows who created some of the earliest minicomputers to IMO medalists who are unbelievably fast problem solvers. I'm not going to say that's a generalizeable statement. It's not, and that's the point -- it's a side effect of spending a career working in teams where people care m…
"Doing research" is not mutually exclusive with doing open source work either. In fact, I think these days, I would be suspicious of a systems software research project that didn't release the code. A lot of systems software research projects use Linux or some other open source code base as a starting point, since re-inventing the wheel is not very conducive to publishing quickly.
It's pretty obvious why companies would want to look at open source contributions. They show that someone works well on a team, is motivated, and can write code. Of course, they're not ever going to be the only thing companies consider.
It's not even that they don't work on projects in their spare time. They do, but they're not part of open source culture, where people share things by default. They're satisfied with having made the thing itself, and they're not going to put their hardware or software project on github any sooner than they're going to make a website for the house they built from scratch.
How do you know they work on projects in their spare time? Is it because they shared them with you?