Yep - that’s because Apple has not released a stable way to create alternative app stores. Developpers still have to find roundabout ways to offer an alternative store. I am sure they will adapt as soon as they can get a stable stand-alone app working in the apple ecosystem
I think you could reconsider your position re Pixel/Google phones. It is the one major maker that allows you to deploy an OS securely on their phone and GrapheneOS is grand. I have used it on a secondary phone for years.