Downhill only MTBers, lack skills. Downhill only MTB are built like tanks, there's no skill to riding one of these over any rock sections no matter how fierce.
The hardware being heavily fortified steel and alloys, is what does all the work. XC bikers do uphill and downhill, and their bikes are far less fortified yet they do much of the same fierce terrain.
These guys make me laugh about how they brag how tough it was to push their bikes up 1600 feet. Wusses...