Because I'm climbing routes I wouldn't have ever climbed (4-5 days a week), to ride DH runs. I climb techy shit that was never even an option, I run EMTB or Boost all the time, fuck that eco shit. I like doing that stuff at 25kph, I don't need the fitness as I get it from gravel, smart trainer or running, its just better fun than riding a normal bike when I'm out for fun, riding a bike for me was never about fitness, I've ridden many epic endurance races and been really fit and the fitness was always a by-product of riding. I still go as hard as ever but running full soft DH tyres and big suspension has made me a better rider in the last few years. Don't be afraid you curious.