Logistics wins a war, I guess.
Double or triple-packing bays doors have a couple counters, at least:
1 - Doing this runs a pretty heavy risk of eggs vs baskets syndrome. Sure, you can hypothetically cream a whole company into a Leopard, but on the other hand, that's trusting an entire company to the armor of a Leopard, with appropriate consequences if said ship gets shot down.
2 - As mentioned, offloading mechs from bulk cargo status is a much longer process than your normal walk-off. Sounds like the perfect time for defenders to hit your landing zone if they're able to, in order to hit your ground forces when they're at their weakest.