All of the major commercial OS vendors are trying to do that. Apple started it with iOS. Google have gradually been tightening the net. Microsoft are furthest away but they have the longest legacy of freedom so they the furthest to go.
Obviously they aren't going to publicly say that's their intent, but you don't have to be a genius to read between the lines.
As for why... money and power are pretty big motivators.
What evidence have you got that they aren't? I don't see why you're expecting some quote from Tim Cook saying "yes we're going to lock down macos in the next 10 years". Obviously not going to happen.
The evidence is their actions with gatekeeper, app signing, removing the right-click workaround, etc.
I remember when there was option to run any application. With Sequoia there are only 2 options: App Store; App store + Known developers. Third option was removed. You can still run other apps but you need to manually approve them with ~3 popups where first option is "move to Bin". You need to do this after every OS or App update. I wonder when this option will be removed as well.
This is tiresome. You cannot lock down development machines. If you pay attention you'll see that OSes made for development work will be the only ones not locked down. Android was a holdout but Google is now tightening the screws. MacOS, Linux, BSD, and Windows are the only OSes that can't be locked down. Microsoft tried but they abandoned that.
Good point, but it is entirely possible for Apple/Microsoft to lock down "consumer" versions of their operating systems, effectively turning the common man's computer into a glorified phone and a new cash cow. Add to that a requirement for an online account, age verification, and other malware.
