So holding ground requires a bit of elevation. By using a hill to hide behind, the tanks can create local superiority massing behind the hill, but other then this tanks are not a good stationary defensive tool. (I think this is true IRL as well)
Holding OPEN ground, on the other hand, is much harder. In a running battle, tanks have vulnerable flanks that have to be protected or they lose mobility. Tanks that lose mobility are easy prey for long range firepower.
Tank formations are best to attack with, not to hold ground with. By moving forward with tanks, you automatically close the gap, and stragglers/tracked tanks are not isolated and killed but left behind for rear logistics crews to tow away and fix. If the enemy is running from tanks, then they arnt flanking them, so the mobility/track/through armor crit issue is much reduced. And if the enemy does stand and fight, the overwhelming numbers of tanks on the attack gives the odds you need to win locally--the stacking 2 tanks to 1 hex kind of illustrates this, as your tank company fits in 6 hexes, allowing for better massed forward fire then mechs who have to spread out more.
I believe the tool for holding ground is infantry in buildings, or infantry in cover in improved positions. Behind this you have fixed guns/artillery, that can attack anywhere from a single point in the rear. Tank destroyers and field guns in hiding augment infantry to protect tight spaces, as a deterrent to attacking forces for moving too fast. They dont really stop an attack, more just slow it down so you can counterattack or have time to land that artillery.