Sure it could be done but it's going to be slower and more unreliable than a centralized solution, and you'll need run it redundantly so you can be sure no one

Sure it could be done but it's going to be slower and more unreliable than a centralized solution, and you'll need run it redundantly so you can be sure no one is malicious. There's also problem of incentive. Why would anyone want to run your builds? So you can run theirs? In that case why you won't run yours and their theirs and save the overhead?