Why shouldn’t I go on my own ?

That is certainly an option, but there are a few things to bear in mind:

• most trails end in a remote area, from which it will be difficult to get transport back into town, let alone a ride back to the top for another run.
• it is easy to get lost: when you’re riding down the mountain at high speed, it’s easy to miss a turn-off and end up at a dead-end. Hiking back up the mountain with your bike is nobody’s idea of fun (we’ve been there)
• should anything go wrong with your bike, your tour guide is a skilled mechanic and carries basic spares for most trail repairs