They can, it would likely just increase the cost of cheap devices to end users, as the manufacturer now has to provide additional software support and does not want to lose money.
Because when you buy most smartphones, you're buying a vendor locked device and choosing to stay within their ecosystem. That's how Google has designed their monopoly. Apple is the same way, but non-fragmented.
You've never had to wait for Dell to type apt update and apt upgrade, but MacOS users have to wait for Apple to update their computer.
These manufacturers gladly took in AOSP back in 2011 when it was still truly a great open source project - exactly as the name should require it to be - and also when security requirements were much much lower. Of course to keep up with device security it turns out you need complete control over the whole stack and regular updates anyway, so now these manufacturers are in a pickle of a situation.
Its possible the forced apps are a cost recouping mechanism. But how does a phone bootloader being locked down become Google's fault? Does it mandate that for some kind of Android certification?
Yes Google mandates a locked bootloader in order to meet Google Play Integrity's remote attestation. More generally it mandates a perfectly clean and valid secure boot chain. Among a variety of other requirements.
One could argue that those “cheap” devices are ewaste from the beginning, and customers needing lower cost mobile devices should be buying more expensive ones used or refurbished.
I never had to wait on Dell to type apt update and apt upgrade.