Because you lose control over your product for those percentage of users. What happens when you want to run an A/B test and a huge chunk or your users is on a platform you can no longer create tests for? What if the A/B test is wildly successful now you can't even ship those changes to a chunk of your users?
They may have a decent number of downloads, but none of us knows how many users they have. I don't know how long the app has been on the store, but it only has 53 ratings which indicates very little engagement.
There is almost no incentive to rate app. They can't reward user and why should someone leave a comment? This is why I like what Amazon does with comment.... but if most of these are real users I think the app did a good job.