Has that ever been explained? I mean why not they're basically single heatsinks that work twice as well, so there really doesn't seem to be any reason for vehicles to be unable to use them.
My personal assumption is that it's a mix of tanks having far, far less surface area relative to volume than mechs do, complicating thermal radiation, and the fact that despite having far more volume relative to surface area than a 'mech a tank will tend to not actually have many individual large internal spots to cram components, on account of having far more people sitting inside of it and needing paths of egress and ingress for all of them.
Something related to this is also my explanation for why missile and ballistic weapons don't generate heat in a CV, and CVs don't normally have environmental sealing: because they have so much less surface area (radiative surface is important), they can't have appropriate thermal management without open air ventilation (the lore reason ballistic weapons on mechs generate heat scale is that even their breaches need a relatively high amount of atmospheric sealing when not firing, so they can be used on airless or toxic worlds). While this sort of less sealed construction lets them trivially vent the firing heat of their ordinary guns, it's not adequate for the sort of heat lasers lay down.
This, I pair with the fact that the first battlemech, the Mackie, was also the first machine to ever carry a ground-scale PPC, a notoriously hot-running energy weapon. At some point, thus, the thermal load of cutting edge weapons grew high enough that a brick like a tank could not achieve sufficient radiative surface to reasonably use energy weapons without giving up the atmospheric sealing needed to be usable in all plausible environments.
The heat budgets of vehicles have even more need to be kept balanced than mechs as well, because rather than in the least likely to contain weapons part of the machine a vehicle has (the head of a mech, usually very far from the hot bits) the crew is directly in the middle of where all the weapons are.
Do the mechanics fully support this? No, but they're also not a 1:1 simulation of existence in-setting.
Probably both to make Combat Vehicles nonviable as well as the baffling size difference usually depicted between Mechs and CVs of the same mass.
The 68 ton, modern day Abrams is 8 meters long, whereas the generally understood range for mech height based on the few glimpses we've gotten is typically thought of as 8 to 12 meters in height. Mechs happen to have most of their height eaten up by noodly little legs and only reach the same mass by having arms to weight them down. Granted, you'd expect the tanks of the 31st century to be physically bulkier than modern ones per unit mass due to the lower density materials of the era, but then again that would also mandate even physically larger 'mechs for the same mass, because their shape still spins the volume outwards rather than concentrating it in a brick. Plus, myomer itself might be unusually low density compared to the motive fuckery that moves CVs, being synthetic protein filled plastic tubes.
Artist's depictions can also be dramatized for shits and giggles to sell a certain impression.
