But that kind of privacy based security model is anathema to Google because its whole business model is based on violating its users' privacy. And that's why they have come with such convoluted implementation that further give them control over a user's device. Obviously some government's too may favour such an approach as they too can then use Google or Apple to exert control over their citizens (through censorship or denial of services).
Note also that while they are not completely removing sideloading (for now) they are introducing further restrictions on it, including gate-keeping by them. This is just the "boil the frog slowly" approach. Once this is normalised, they will make a move to prevent sideloading completely, again, in the future.
