That other something might be led by a dickhead that you can't get along with, or a community that's not receptive to your suggestions or patches.
Social processes and blockages can lead to losing motivation to contribute. Not everyone is cut out to be this happy-go-lucky team player that has no personality.
As for myself, I'd rather start from scratch because 1) I'll control every variable, 2) I don't have to mess around with some existing social and tech infra, 3) I don't have to talk to anyone and convince them my ideas are good, 4) who wants to spend more time arguing or discussing than writing code?
The solodev experience is just better. You're not hamstrung by politics and social process. Being a team player has only led to misery for me.
Collaboration can seem daunting and hard work, but it's work that will yield results which is time better spent than that devoted to projects that end up abandoned.
I'm not convinced these efforts to put everyone's ideas in one place and one software are good. Often, design disagreements arise which are incompatible. Under this model of "everybody love everybody" lameshit, if you can't get the group to accept your idea, it doesn't happen.
You don't run into this problem when you write alone.
But if it makes the original author feel good (everybody loves themself?), then it's OK.