I think #1 reason nobody even tries this anymore cause it dramatically reduces chances to win TDF and this is main objective for all teams with dominant GC rider. Nowadays to win TDF you must be at 100% and it's virtually impossible to do after the GC battle at Giro.I maintain that the #1 reason that we haven't seen the Giro-Tour double is that only one rider has actually tried it while at the peak of his dominance, and that was Contador doing the sickest Giro route in 2011 and then he proceeded to crash hard in the Tour and lose 1'30 doing so before blowing himself up trying to get that all back.
Contador had a window to do it. Froome had a window to do it. Pogacar had a window to do it before. I believe it's entirely possible. Both Vingegaard and Pogacar can do it IMO
We may dream about it and say Pogi and Vingo can do it, but the real question if they WILL. So far nobody did that.
Vingo with his insane recovery has better chances for it IMHO, but untill he crushes TDF we won't see it, cause Visma won't risk TDF win for Giro-TDF fairytail.