Is it just me or does it seems very odd that GrapheneOS only runs on a phone produced by the company that makes Android. Meaning that ironically, it isn't a Google alternative.

I know the reasons are technical, but still, it means I have no interest in it as somebody who is actively de-googling myself.

The reasoning is explained here: https://grapheneos.org/faq#device-support

Note that Google is in active talks with an major Android OEM for a few months already to help them meet the requirements for a subset of their future devices. They are very optimistic about that.