Nobody has mentioned one of the top purely technical reasons companies are reluctant to open source things: support.

I worked for a company that opened a UI design framework and people loved it, but the moment you have an outside audience, you can't just make breaking changes or pivot the direction. You have to be sure your thing is completely stable before you open it up.

They felt they couldn't move fast enough while supporting the open one, so they forked it and just maintained the public one so the private one could change faster.

There are costs to support. I'm not saying companies shouldn't do it (Google does, all the time), bit smaller companies may not be able to afford it.