Linux is not security hardened. It does not properly sandbox applications (and there is nothing as secure as android's sandboxing on linux). In fact, most linux package managers do not feature any sandboxing of applications, period. Linux does not implement verified boot. It does not harden against physical port attacks. It does not use a hardened memory allocator. Privilege escalation is simple because of how straightforward it is to compromise a wheel user (sudo user). Linux does not harden it kernel flags by default. Alpine (and most linux package managers) are not secure (aka does not pass the TUF threat model). Most linux distros dont feature a read-only root filesystem, which would help to improve security. Also, Systemd is a bloated init system and has a massive attack surface. GNU's tooling is also bloated and freebsd's would make a good alternative (like what is done by Chimera Linux)

Here are some readings on linux security:
Article by one of the Whonix Devs https://madaidans-insecurities.github.io/linux.html and also are hardening guide from them https://madaidans-insecurities.github.io/guides/linux-hardening.html
Wiki page of Whonix considering many linux distros for whether they make a good base for Whonix's security distro: https://www.whonix.org/wiki/Dev/Operating_System#Alpine_Linux
Kicksecure's wiki: https://www.kicksecure.com/wiki/Documentation

Here are some Security hardened distros (Note that none meet the threat model for a mobile phone OS as they dont feature verified boot):
https://www.kicksecure.com
https://github.com/secureblue/secureblue
https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/profiles/hardened.nix.

Special mention which isnt hardened but has great potential: https://chimera-linux.org/