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.