First off, not everyone has or even needs a 4K TV. And if you don't have a 4K TV, you don't really need a 4K Apple TV.
Furthermore, the 4th Generation Apple TV (aka Apple TV HD) still runs the latest tvOS release. Who knows for how much longer, but, at least for now, it's still supported. Also, if you're not using Apple Arcade and/or playing other games on your Apple TV, it's not like the A10X Fusion, A12 Bionic, or even A15 Bionic versions of the 4K Apple TV are even necessary.
Past that, people generally hold onto old Apple TVs because they think they can hack or jailbreak them. 2nd Generation Apple TVs were great for that. I'm not sure if anyone has gotten that to work with either the 3rd or 4th generation models. But considering that, unlike any 4K model that has ever come out, those ones still have a USB port; it's at least something that can be attempted.