I think a big part of this is that the long-distance missiles, when all of this was invented, were not very accurate. Sending 10 to do the job of 1 might have been necessary just to hit the intended targets.
Modern missiles are quite capable of precision strikes.
yes, this is also why yields have dropped considerably. I'm not sure if there are any > 5 MT weapons in the US arsenal anymore. I think most are in the 500-750KT range, the missiles and delivery vehicle are accurate enough to produce the same result as the larger warheads and 5-7 (can't remember exactly) warheads can be carried by a single missile. So instead of having one missile launch one giant warhead that may hit 50 miles away from the target you have one missile launch 5-7 smaller warheads that hit within 50 meters of 5-7 different targets.