"Principle: armor in strength produces decisive shock effect
The psychological shock effect that comes to troops on the receiving end of a massed armored assault is terrific. This effect radiates from the point of attack in concentric semi-circles, as do the waves from a stone dropped in the water near the edge of a millpond. If the attack is in strength, these shockwaves reach to the enemy division, corps and army headquarters. Shock effect gives armor part of its protection and hastens the disintegration of the enemy force attacked. The shock effect of the mass employment of armor varies as the square or cube of the number of tanks used. Attacking with armored strength too small to produce decisive shock effect often results in great losses and inconclusive results."
https://www.benning.army.mil/armor/earmor/content/Historical...
Simply put, they are pants-soiling terrifying spectacles of death and they prove overwhelming. Once significant momentum of a well-executed armored attack has been gained, they are difficult to slow or stop. Drone swarms may make this tactic obsolete, but we're not there yet.