That’s because most of the 7.62X39 shot in them is cheap Russian Wolf and old surplus ammo. Accurate it ain’t The 7.62X39 ain’t no slouch itself. Various bolt-gun makers chamber it with good results. Heck the 6mmPPC widely regarded as one of the most naturally accurate cartridges ever made is a kissing cousin in design to the “Russian Short”. Also Ak’s aren’t accurate because of their designed loose tolerances. The good side of this is it allows for their legendary reliability.
As for the quality of Wolf and other surplus ammo out there, it seems strange to me that one particular cartridge would do so poorly. The 7.62x54R is no slouch when it comes to accuracy and although I've never owned a 5.45x39, people that do report that AK's in that caliber generally shoot 2 to 3 MOA.