They don't, because in general cash players suck in MTTs big time. Same thing other way around as it requires totally different style to be succesful in either.
Just look at Gus Hansen. Beast in MTTs during his time as his style was dominating but terrible at cash games where people weren't afraid of losing their chips as they could just buy a new stack.
You could just say that live cash player is the weakest poker player on Earth and it is just as true as other way around.
I agree that the greatest downside in MTTs is huge variance. It is beatable though with good game selection (1st rule is NOT to play in any Stars.com NLHE regular tournament). In live tournaments you virtually never beat the variance so yeah, I at least acknowledge it and play them purely for fun and entertaiment (I mainly play just when I win packages easily and take it as a vacation).
Except, the whole point is winning, consistent cash game players generally aren't interested in becoming serious MTT'ers but almost to a man could quite easily make the switch, while the opposite is almost certainly untrue.
MTT play is simply much simpler, period.
As much as tournament players want to dismiss the idea, as someone who has played a decent amount of poker in the last decade, I can tell you it's crystal clear that the live MTT player is the weakest player on earth by category....and it isn't that close at all.
The canard about the two modes of play being "completely different" is just a cheap out MTT players use to delude themselves to the inescapable fact that MTT play can be simplified much of the time due to stack size.
Also, there is no evidence that Gus is a top MTT player anymore. The game has changed so much from when he dominated that I would be surprised if he was even a top 100 NLH mtt player right now, and wouldn't be shocked if he wasn't a top 250 player.
Now, that doesn't mean he isn't brilliant and courageous and that is taking NOTHING away from his past accomplishments.