AT guns are finishers against tanks, no 1 man army counter as they are way cheaper.
An AT gun deals consistent high damage with (usually) good penetration. The AT gun simply is not mobile.
Shouldn't you need a inf squad nearby to snare the vehicle? An AT gun is a SUPPORT WEAPON, that means it is SUPPORTING other units. A tank with an engine damage is defenseless against an AT gun that faces it.
I wouldn't explain it better
