Full backs are supposed to stop the crosses from their own side and tuck in around the centre backs when the threat is coming from the opposite side. Also, they need to be able to defend the far post and be aware of this threat.
What we are seeing is a direct result of not having natural fullbacks in those positions.
Total incompetence.
In the age of the wingback I fear far too many fullbacks fancy themselves more as attackers than defenders. Coaches encourage them to get forward and I reckon there is less emphasis on the quality of defending. Your typical footballer's brain capacity is small and finite and, once you've set aside space in there for fashion, crap music and tattoos, there's only so much he's going to be able to store about tactical nous and defensive prowess.