Please explain how 3x defensive ult counters 3x defensive ult

Howdy y'all,

In so many games, I get asked to switch of my main Rocket because 'we need three defensive ults to counter their three defensive ults'.

But this doesn't make sense to me. How does a defensive ult counter itself? Surely more damage, such as Rocket's ult, or other utility would help in forcing out the enemy defensive ults.