This one is.... Weird.
Mostly it's for money, other times it's because they can't really get any HELP on a project, sometimes in control and other time's, the team is just at each other's throats with piss, vinegar and enough rage to slaughter a field of.... I guess, just insert something to Slaughter here.
I knew a 2 man team working on a Horror game together, one was working on the art while the other was working on the coding. One of them use to be a roommate renting a room from me and he refused to get a real job outside of coding as his Teammate was paying him for the coding.
He got kicked out because he fought with the teammate over the direction of the game they worked on and got pissed it wasn't going to make him famous, just the artist who made the thing. When he bit the hand that fed his pockets to keep him in the house, he got kicked out and it became a tale of both Control and Power.
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Trust me when I say NOONE in indie WANTS to work on a game SOLO; they do seek out others to HELP them; but everyone wants them greenbacks, cash, paid work and it boils down too 'who is willing to work until it make's it big and who want's all that money NOW' or 'Who is willing to work on the project and not throw a fit when nothing goes their way?'.... and very rarely 'Who just wants to help work on a passion project without pay?'