Probably because he doesn't.
CM's dropping in to cover for full-backs is an entirely normal thing at every level of football across the planet. Would be stupid not to have someone fill the gaps when full-backs go forward otherwise you just leave yourself wide open to overloads on the counter. Other times, like in the PSV game, Raskin & Cifuentes would drop in to help Tav/Barisic double up on Lang/Dest or Bakayoko/Teze.
Our attackers don't do the job of centre mids. The closest it comes is Cantwell going between an 8 and a 10. The likes of Dessers aren't anywhere close to 'doing the job of CMs'