They don’t?

I’m pretty sure you can install iOS 16 on an iPhone 8, which came out in 2017, almost 6 years ago. And that’s a major system update. If you just need security updates, the latest one was in January and supported phones as far back as the iPhone 5s, released almost 10 years ago today.

But in reality, people want better phones and better cameras every few years, so they buy them. And they tend not to throw out their old ones, but sell/trade them or pass them along to someone else.

Hell, I have a laptop that's over 10 years old. It isn't officially supported on Windows 11, but I'm sure I could get it on there in some unsupported way, using Rufus or another tool that removes the TPM requirements and have it be usable and secure. It runs Windows 10 without complaints. I can run an up to date Linux distro on it and be completely up to date and secure. So, like you said, why can't phones do the same?

I have a ten year old iPad that i pretty much only use to watch TV in the shower. It just became incompatible with Hulu last week due to the iOS and I’m super frustrated by that. I can still use it for Netflix and paramount + but i was in the middle of several Hulu only series!