Does no one yet make a decent full length guard for disk braked bikes without mounts?
Are you asking why there are no decent full length mudguards that fit bikes with disc brakes, and the bike itself does not have a mounting point? If so, you can get clips that fix to your frame to create the fixed mounting point that the mudguard can then fix to.
Or are you asking why there are no decent full length mudguards that fit bikes with disc brakes, and the mudguard itself is designed in such a way that it does not require there to be any additional mounts within the bike frame for it to fix to? For example it just affixes to the crown and seat stays of the bike via an elastic strap? If so, then they will be flimsy wobbly things.