I know that the steppes weren't particularly hospitable either but it seems like at least in terms of grazing for caravans' horses and beasts of burden as well as water sources the steppes would be easier to pass through than the mountains and deserts of central Asia and the middle east.
It's important to remember that very few (if any) traders travelled the entire length of the silk road- it was more like a relay of different traders.
The central Asian deserts were already populated by nomadic people with a long tradition of trade, which facilitated the establishment of the greater trade routes in the way that they were.
It's also worth noting that primary east-west routes went through the sea, so you probably mean land routes.