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.

Why does the manufacturer of the phone have to write os upgrades?

I never had to wait on Dell to type apt update and apt upgrade.

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.

That was my point, this is a self created problem by the manufacturer.

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.

No, it's a prerequisite to doing business.

The OEM phones are cheap because the manufacturer sells them at a loss, recouping money by locking them down and pre-installing certain software.

The alternative is that Google is properly regulated, or cheap smartphones phones don't exist.

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.