There's a lot of reasons. Building a closed guarden that locks people in feels a lot safer from a competition standpoint than the alternative, if you can make it hard to steal your customers and only a few (always remember that being on and reading HN means you're not much of an average user), it's also just easier than being open, a closed system can make changes much faster than an open one that has to get concensus before adding a new feature for example.

In this specific case and most of the time where Google's complete lack of support shows up it's because having manual review costs money and the process that screws the app developer is easier than having a secondary process that needs more people to decide if the first automated or rote manual scan made a mistake.