Apple

Because Google already lets apps do anything they want no matter how malicious. There's no reason to leave the Play Store.

Apple has people sneak past their rules on occasion because screening is hard, but they have and enforce rules that protect your privacy that malware companies like Facebook don't want to follow.

It absolutely has happened on Android. The Russian government has launched their own app store, as an example of a state-owned-and-operated third party app store.

Additionally, once both iOS and Android are opened up, the capability to control the end-to-end distribution on both platforms simultaneously becomes a much larger incentive for major corporations; gone are the days where some users receives some features earlier because the other app store have not pushed the update yet -- they control it end-to-end.

I mean, I should be abundantly clear: simply operating a third party store does not equate to malicious intent. Some would argue the corporation case above could be considered beneficial for users. However, having third party stores with varying degree of security capabilities increases attack vectors for bad actors, and thereby making it more difficult for everyday users to manage -- an additional layer of complexity iOS users have not had to deal with for many years and very very few has signed up for.