The problem with Linux is that it has very poor security compared to Android (which is built on Linux too). Nobody would want to use this on a phone. Linux needs a lot of work to reach Android on the security level (and usability on phone screens too).
Discussion
Yes, Linux security is poor
Security is important but should not be our sole focus. Freedom is actually more important. #GNULinux on mobile provides a reasonable level of security for most people with average threat models, and it is gradually improving even with a shoestring budget compared to Google's and Apple's.
Usability on phone screens is pretty good a lot of apps, and #phosh mobile settings can force apps to adapt to smaller screens. We can expect major improvements if people invest in alternatives instead of being being locked in to what Big Tech has to offer.