why open source friendly phones are so expensive ?

Not really. As you note yourself, there's virtually no difference between a phone from six years ago and a modern one, except that the new one is larger, has a large camera bump, and has some pointless AI crap included.

If you buy a cheap phone once every ten years and keep it until it breaks, then you're not generating any significant revenue for the industry. You're a bad consumer.

If you buy a new, expensive phone every two years, and contribute to filling landfills with expensive e-waste, then you generate "value", and you're a good consumer. Bonus points if you produce CO₂ by using a cloud-based "AI assistant" rather than checking your calendar yourself.

It is in the interest of the hardware companies to coerce you into being a good consumer. If cheap, robust phones from a few generations ago were supported by recent versions of Android, then most people would probably prefer to keep their old phones.

> when computers are mostly identical with no real improvement

That's an interesting question. Probably because the computer companies do not control what software you run on the hardware they produce: my desktop is eight years old, and I use it to run the latest Debian, whether HP like it or not. (I don't know if I could install a recent version of Windows on it, but I'd probably manage.) Not so with my smartphone: I've stopped getting software updates a couple of years ago, so now I have to run LineageOS, which means giving up on banking apps.

(Microsoft have been trying to obsolete older hardware by requiring a powerful NPU for their AI features, but for some reason they couldn't bring themselves to obsolete older computers altogether.)