Ah, the lock is not something that check the license and tells "oh no you're not allowed". It's a combination of not having something, and complexity to add it. Sometimes, that's because you need something tricky. Sometimes, that's because the project lacks any documentation, is written in a pretty low-level (verbose) language that obscures the high-level picture, and it's hard to figure out how it works. Sometimes, that's because FLOSS != KISS, and there is a lot of FLOSS software that has its own unique proprietary protocols and data formats, and does things in a manner that's completely incompatible and non-interoperable with any other system. I have no clue about MySQL or PostgreSQL internals (actually, I heard a word they're good in this regard), but is the case for various FLOSS projects - I've dug into some and cursed their developers.

If you overcame that and made a patch, but it's not accepted by the upstream - there are maintenance costs. How long would you be able to maintain your own fork of a large software project, keeping up with the upstream? I've tried with a few small ones (say, at a small ISP company, I just needed some custom pppd patches for in-house billing logic), and found it to be not a pleasant experience. I believe it's not just because I'm a lazy ass - I saw tons of forks on GitHub that were slowly rotting away, maintained for some time but eventually forsaken by their authors and far behind their upstreams.