It's most likely because the bike can't move around under you when it's locked into the trainer. The saddle pressure is focused onto one specific spot, which wouldn't happen outside.
It's why rocker plates have become popular for some.
This wouldn't happen with the previous setup, which wasn't even right for me since I was using someone else's bike/turbo setup. It would be uncomfortable, but not even remotely as much
I get that on the turbo but on the road too. All it takes is an about threshold long effort and little movement in the bike to go numb.
I just move in the saddle. If I'm in the turbo I'll get out of the saddle for a minute.
It's most likely because the bike can't move around under you when it's locked into the trainer. The saddle pressure is focused onto one specific spot, which wouldn't happen outside.
It's why rocker plates have become popular for some.
https://zwiftinsider.com/rockit-launcher/