The idea behind the fairphone is that it's made fairly. It looks overpriced because they're paying a fair price for the raw materials + production costs. If other companies didn't exploit third world countries their phones would be priced similarly to fair phone.
You don't buy fair phone for the specs, you buy it so you can be certain some child in south Africa didn't have crawl in a mine to get the the metals that go into phones, or have a child sit in a factory putting together the chips that go into phones. Or you buy it because you don't want to throw your phone away after 3 years because you couldn't replace the battery or the screen or the charging port.
But where does the new hardware come from? Google has one OS that is the same over all Pixel phones, it doesn't stop them from churning out a new Pixel every year. It also doesn't solve the problems Fairphone aims to solve which are a) ethically sourced materials and b) reducing ewaste by having higher repairability.
Let's say they create their own OS. How is the OS going to make sure the underlying hardware is "fairly" acquired? It's not. Nor is the OS magically going to turn a non-repairable phone into a repairable one. That's the reason why Fairphone makes their own phones, so they can verify their materials are ethically sourced and the phone is repairable.
Actually the company recycles its phones. If you don't like your Fairphone you can send it to them and based on the model and the state of the phone they'll reimburse it. And how long is long because Fairphones are over a decade old?