It's easier to start again than to build anew in a lot of cases. You'd need to pay me to stare at and figure out existing code; few people (especially nowadays)

It's easier to start again than to build anew in a lot of cases. You'd need to pay me to stare at and figure out existing code; few people (especially nowadays) have the attention span to invest 6 hours in fixing a tiny glitch they deal with every 2 months, but such impulsive development/drive-by patching is what fuels >50% of OSS development. As for the group thing, you're basically describing distros there. Commercial vendors provide value-adds such as security patching and corporate-focused features around a community-derived base system.