I don't think this is actually true. Phone makers seem happy to have the platform locked down even further. They're putting even more roadblocks, as evidenced by most of them making bootloaders unlockable.

They're happy that it is locked down. They're not happy that they're not the ones doing the locking down.

Any discussion about this is shut down. I just read an article https://www.bbc.com/news/articles/ckqxvy98x578o In the UK according to the BBC, politicians are pressing Samsung and apple to make stolen phones bricks. Samsung in response said that they have implemented several security features for this purpose. I think Samsung considers removing OEM unlocking a security feature according to the bootloader unlock hall of shame.

Intel used to do the same with their anti theft technology aka Intel theft deterrent on their classmate PCs, so students had to enter unlock codes from an IT admin every 3 months or every say 200 boots to prevent the computer from being locked at the bios level preventing it from booting an OS, and there was an update which disabled that functionality permanently. And you would be warned when you had around 50 boots left to get the unlock code.

Making the bootloaders unlockable is a good thing.

This is what lots of people forget.

Android is not a Linux desktop or server distro. It is not about you getting to put what you want on the hardware you bought for free.

Operating systems are hellaciously expensive to maintain and that only gets worse in markets with rapid hardware improvements, as the smartphone market was up until maybe the late 2010s. Google is not a charity. SV did not come into national prominence for making investors $0. There is no money in giving maybe one in one-hundred smartphone users (and that's being generous) a bunch of code, for free, so that they can put it on their gizmo and talk to their nerd friends at their hacker meetup.

When they pitched Android as "open", they meant that carriers and device makers could load it up with all of the revenue-enhancing bloat that they wanted. In return, Google got a device that would let them hoover up all of the data they could ever want in order to build better ad service profiles for those using the devices. That is, after all, their business.

For a while, this could coexist with us screwing around with a real-life tricorder. At some point, though, the free stuff turned into a revenue opportunity that had to be exploited. And so, it will be.