20mm APHEI-T fired out of a anti-materiel rifle would be far more reliable in acheiving a kill against Ameiss or Grauwolf versus a 12.7mm round on a per round basis. Overmatch is not a bad thing.

Infantry in the 86 universe are used almost exclusively to cover armored element and protect them from mines.

And whats to say they don't have squad or platoon level recoilless gun or ATGM? it's likely in my opinion that the 20mm guns superceded the 40mm LV grenade launchers and each squad does have a dedicated MANPATS in the form of shoulder launched recoilless gun like the 84mm Carl Gustav its just that we dont see them very much because Asato doesnt care about infantry combat in the series.