I don't think you can fit a 50m gun into a truck.
Or a nuclear reactor...
Kray, the reason they don't nuke each other is the same reason countries don't nuke each other in the real world - it's a horrible idea for everyone involved.
They have literally hundreds of guns. You seriously don't think they have long range ballistics?
You also can't really hide tanks or larger with the amount of battlefield surveillance that goes on.