Part of it is greed, of cours. Wanting to "rake it in," and buy themselves that "solid-gold jet pack" (Dana Gould reference). That's humans for ya!
But in most cases it's probably the greed of needing to survive in an independent business where the competition is extremely high. "We spent all this money buying this equipment and renting this space and paying for the concept, and paying for the talent, etc. and we're up against companies like Disney!" That's why there are a lot of contracts that are one-time deals--they pay the talent for their work in the development stage, and that's it, they don't have to worry about paying anyone later. Those are the projects that the non-union talent off the street migrates to. But SAG-AFTRA talent, the "real" and protected talent in the industry, requires much more attention... since the company is paying for the names and the established careers. For the professionalism. For the status of being a SAG-AFTRA project.
No company knows if their game is going to be a success, or a bust. So for some, having to pay out regularly beyond that first appearance fee, when the company wants to save up money for future productions, is a problem for them. They've got to budget themselves, not just for that one project but for the ones that follow, if they want to remain players in the game and have a future doing what they're doing. But if they're hiring SAG-AFTRA to raise their project's status? They should have to pay for that privilege.
Yet right now, nobody from the big companies to the upstarts are giving the actors their fair share because the entertainment media has changed since the '90s, and they're not contractually obligated to.
Another concern for companies is piracy--someone stealing an idea, releasing it before the original company does, and then taking the credit while then turning around and saying, "You stole my idea!" because the original concept came out later. Hence the lack of description to everyone, even the voice talent, as to what they're getting into. (Which is a little beyond paranoid. If you're doing a project, surely you have it copyrighted already!)
When Dwight talks about not wanting to mention things he's appeared in, that's a third problem: crappy material. You walk blind into something and hope for the best. Having done non-union acting, that's part of what you're faced with and have to deal with; you get a job, you go, you shut up and do it. (That's how I ended up standing in total darkness at midnight, no food, no water, no lights, no heat, waiting on a dirt road for a bus in the middle of the desert at some unmarked crossroads, to come and pick me and a hundred of my new friends up from the day's commercial shoot.) This kind of blind hiring is not acceptable for SAG-AFTRA, however, because as a professional actor, you want to build your resume and have good, meaningful roles--not reciting crap lines or finding yourself in a role that goes against everything you believe in. But... ethics? In a land of profit and publicity? Banish the thought!
"They sell a LOT of tin foil in your town, don't they?"