Does it matter? The Russian parliament, most of it's wealthy citizens and all the critical industries would be dead.

Like...that's the whole point of MAD. You might as well ask "well why didn't the US and Soviet Union go to war? Obviously they won't launch..."

The entire point of a nuclear deterrent is it stops people asking those sorts of questions, because ultimately it's a gamble. The only guarantee you get is "don't invade my territory and I won't launch". As soon as you start not doing that, you get to ask if you think it's a 1% chance, or a 2% chance, or or or... You get to find out when your capital and most populous city do or do not explode. And you get to roll those dice over and over again.

Like I said: what % chance of Washington DC, and New York City being obliterated would you take for a chunk of Mexico? Obviously if Mexico launch you're going to blow the hell out of it's capital and probably some other targets, but you aren't going to be doing that till after those two cities are gone.