Fortunately the community doesn't have the perverse incentive of a hardware manufacturer. If updates stop flowing it encourages people to buy new devices.

I'm not talking about "the community". There are multiple manufacturers/distributors of Linux laptops. They do nothing to stymie the flow of updates, even though your assertion about selling new hardware would theoretically apply just as much there.

It's a good comparison. I would speculate that it's the target market. If you're buying a Linux laptop you are likely very technical, and know about and want the latest updates. I doubt System76 would get many sales if Pop OS was based on Ubuntu 20.04 and wouldn't get any updates.

However with Android, companies sell 100s of millions of devices to consumers that are more concerned about price point than updates. There just doesn't seem to be the incentive for OEMs to care about it, if the majority of consumers refused to buy devices based on old OS versions or ones that didn't get updates, those devices wouldn't get made.