This gets brought up several times in this discussion. Hardware with drivers, firmware and device trees exist, see e.g. Fairphone. It even works with open source camera apps (e.g. done in /e/OS). The problem is not (only) the hardware, it is that the Linux desktop stack is wholly unoptimized for phones. And the fact that Meego/Maemo/etc. are now almost two decades ago shows that it is not 'would be easy to solve'. Heck, Linux laptops, which have been available since forever still have far worse battery life than Windows or macOS.

You need a stack that is designed from first principles for phones. Luckily we have one that is open source (AOSP).