Because 99.9% of population doesn’t know what sideloading is, let alone where to find apps and how to sideload them. So, realistically, there isn’t a need for it. Also, Google apps tend to be free with IAP, so no matter where the app comes from, it can still generate them revenue. On Apples side, their apps are mostly paid once upon purchase, so enabling sideloading would effectively cut revenue.