When a device manufacturer makes a phone that uses android they can do a lot of stuff with Android without any issues (beside UX), other than messing with APIs and legal things. iOS is exclusively made by Apple and nobody can release their own iOS, so they always have prevented others (even the carriers) from putting their own apps on the phone before the user gets the device.

Could google change the terms and say that device manufacturers can not preinstall custom specific types of apps or services?

Why would you buy an LG versus a Samsung? Too many android manufacturers to compete on just hardware alone. Even less so than Windows laptops and PCs. Customizing Android is the differentiator.

I personally hate it, but I understand why they do it. I just wish we could get a class leading vanilla Android phone. Pixel isn’t it.

They've made multiple attempts to do this.

Manufactures really hate the lack of flexibility and there is a small amount of justification: new hardware often does need new controls.

And of course many people like the flexibility of Android.

Because the comparable phone to iOS phones aren't "Android phones" but "Pixel phones". Phones where the OS vendor also builds the hardware and controls everything.

It's just a different model. Single corporation stack gives you more consistency and less bloatware and OEM based stack gives you more market choice and greater accessibility.