Here's what I think it boils down to: working on a codebase with your coworkers is (or at least certainly should be) an inherently collaborative process. On the other hand, a job interview is, in a sense, inherently antagonistic. No matter what shape the interview takes, these people aren't your friends, they aren't your coworkers, they are gatekeepers.
I already have a job as a programmer. At work, I can push back on my coworkers and debate the merits of various designs until we all reach a consensus. But with the Teleport interview, there's an inherent power imbalance that makes that impossible: "I'd really like to argue about this, because I don't think I agree, but I'm afraid that will decrease the chances of them hiring me."
And the only people who are in a position to change this process are the ones who have already gotten through it successfully.