Having to carry around something extra to stack cabs would have been a real pain. But of course maybe Mountain could afford it... or afford to replace the occasional cabs that... well, bit the dust.
Back in the day, there was really only one company that made amps specifically so they could be stacked.
Although the head in this pic appears to be simply resting on the cab, notice there ain't much room between them. Yes, the head has "feet".
The feet of the head sit in metal inserts that look like little cups on the top of the cab. There is another set of these cups on the top of the head. It was all set up so that the feet of one component would fit inside the cups of another component - so it locks it together.
Then again... chains would probably work too!!