At the end of the day what are these open source projects you contribute to used for? You could very well be helping out the exact same parties.
However, there are startups around like Ghost, Matrix, who are doing all work in open-source
Also when you license under GNU, those parasites are scared off rather quickly
The truth reason more startups don't that, because they have no leverage, besides the code and/or want to protect their moat at all costs, so they can brag about "we own the IP" when investors are asking "What makes your offering uniquely possible?"
Luckily, as serial™ (co)founder i know the game well enough and never went beyond a seed-round