ATMs are "efficient" in BV only because they're inefficient in weight.
Efficiency has got nothing to do with weight, and hasn't since BV1 was invented. If something is so heavy that you can pack less of them on a mech, then thanks to BV you'll wind up bringing more mechs or heavier ones.
You're making the same argument IS players have made for 30 years that Clan LRM's are broken because they weigh half as much. Sure, that means you can carry twice as many, but it means your mech will cost twice as much. Clan or IS, you still bring the same number of launchers to the table for the same BV.
Assuming average cluster rolls, and including the BV for 1 ton of HE ammo, all ATM's cost 9.1 BV per point of damage. LRM's with ammo are about 19.5 BV per damage (18.9 with Artemis), and an ERML is 15.4. Yes they both have range advantages, but that can be dealt with by mounting the ATM's on a platform with mobility, and an ATM with normal ammo is just 13.7, still less than the ERML for the same range. The only weapons cheaper in BV/dmg then an ATM(HE) are ERSL and the various range 3 weapons.
But getting back on topic, the question was why not use an ATM. The answer is there's really not that many reasons not to. Yes, I'm a fan of them. Mounted on fast medium mechs they suit my preferred cavalry playstyle quite well, and they also pass the MathTech test.