Hardening Linux to a point in which it's actually secure is well out of the range of normal users and would take a team of skilled devs to have the knowledge and put in the time, no standard desktop OS devs have done this so far. Just because you harden a few SELinux policies or use a distro with a MAC framework without strict enforcement and policies or whatever else you consider hardening, it doesn't fix the inherent architectural problems with Linux and it's overarching security model.