It is really a combo of the cartridge design, the ammo makes, and the guns firing it.
There is a good reason that bench rest shooters do not flock to tapered cartridges. The accuracy potential is just not in there, but then again, the cartridge was designed to promote very reliable feeding, which it does with aplomb
Second reason is that the crap-azz ammo makers are not only using .308 to .311 and variation in between in ammo manufacturing, but some of the filler crud they put in the powder or what they call powder is iffy at best. I have seen foreign ammo from Russia and Egypt and similar places that looked like black powder in not only terms of carbon fouling, but also in it’s actual composition. No joke, I one saw some from a certain country that looked a heckuva lot like wood fibers!
Third reason is that the SKS, the AK, and Mini are not really known for super accuracy. The first two are function-first, designed during a time when it was more important that the gun function in a variety of environments, held by people were less than literate and were more likely to treat the thing like a farm implement than a rifle. The Mini is a good design, but is again not known for accuracy on the AR level.
One of my buds has a Ruger M77 boat paddle as described above with a Leupy 6x33 and he uses it to great effect. If you can find a barrel with a .308 chamber and build up an AR, you get MOA accuracy. A friend is doing that now with a barrel from McGowan, but he will be the first to tell you that feeding cn be iffy, which is a prime reason for development of the Blackout
At the end of the day, the Blackout is the US version of the Russian M43 cartridge, offering all of the good with none of the bad, and really is the reason for it’s popularity, I think
