The problem is that Apple and Google are not trying to give people the ability to download the software they want without catching malware, but to make a profit. If Google were trying to provide that service they would not ban ad blockers from their store. They would not charge arbitary 30%, but whatever operating a software repository actually costs.

The problem here is not a lack of competition. Ultimately, there needs to be one default software repository that people can trust -- you can't have a free-for-all here for the same reason you can't have a free-for-all for software in general. The profit motive is the problem.