Same with people buying Google Pixel’s and then removing the stock Android. Isn’t the Pixel’s hardware rubbish, and the only reason to buy it the software?

Because Pixel allows the removal of stock android where some phones, like Samsung's, actively prevent it.

But you're not entirely wrong. There are non-Pixel phones with better hardware and unlockable bootloaders. Often it's a preference thing. Though it should be said, the more popular the phone, the more likely it has robust support from the custom rom community, and Pixels are popular.

Also, "stock Android" doesn't mean "everything is accessible and configurable" Android. They may put a Pixel-based rom right back on, just a version that's not so restrictive. Many don't need a reason to use custom roms beyond just having more knobs and levers for their phone than stock allows them.

Honestly, one of the major reasons I use Lineage, beyond the big obvious reasons, is solely to keep the "Hold back button to kill foreground app" function that stock android removed long ago.