My guess is that proper XC race bikes are optimized to climb without much concern for descents. IME, as a rule-of-thumb, you spend twice as long climbing as you do descending, so losing a bit on the descents is worth it to gain a bit on the climbs.
Spot on really, so long as you stay on it the climbing time will pay you back, I miss my old XC bike to a point, that point being that I never took it out unless I was getting sorted for a race and wanted to get my eye back in. It would much through the miles so well and if you were on your game get down or across a lot more than you ever expected just with a much finer margin for error. That part was fun to a point!!
Hence evolution of the not race orientated short travel fun bikes that are coming out