This is the future; partially fuelled by malware, partially fuelled by the desire for platform control, and partially fuelled by government regulation.
They tried to pull a similar move with WinRT/UWP, but nobody wanted it, so now you can continue with Win32.
They would love to do so, but legacy compatibility is a major business advantage.
Desktop app development gets increasingly hostile and OSes introduce more and more TCC modals, you pretty much need a certificate to codesign an app if you sideload (and app stores have a lot of hassle involved), mobile clients had it bad for a while (and just announced that Android will require a dev certificate for sideloading as well).
edit: also another comment is correct, the reason it is like that is because it has the most eyes on it. In the past it was on desktop apps, which made them worse