There's a few ungoogled privacy OSes. Graphine, Lineage, CalyxOS, Plasma, and a half dozen others. They're very model specific.
There are also mobile Ubuntu and a "Linux phone" setup.
The problem with Linux phones is that, AFAIK, calling and SMS are not supported. Which is kind of important for a phone to do the basic definition of a phone. In the privacy comms, people who have used them flatly say that a Linux phone is no where near ready to be seriously used by anyone as a real replacement for Android.
You're not wrong that a more universal phone OS needs to be out there, but since the hardware varies so oddly by manufacturer, devs can't rely plan for new drivers and test well. That's why Graphine sticks to ONLY Google Pixel hardware, to keep the driver set smaller and easier to test.