Why would we have seen it on Android? That's not an especially profitable platform. Nothing compared to iOS, in fact. Completely different business altogether.
Can simply look to the Mac. Most of the top apps for Mac are not available on the Mac App Store at all.
You are looking at where the Play Store ended up and what it costs. Others are looking at behavior. The "nefarious" aspect of "sideloading" is not about cost to purchase an app. If you are marketing your app it pays to distribute your POS.
I don't agree on the Mac Store - we are talking mobile, not desktop/laptop. Very different market.