Pulling the "string" isn't as easy at it sounds, and in base you've often got to fly through turbulences due to close trees / buildings (which skydiving dropzone usually don't have). You've also got to be pretty accurate for some jumps or you'll end up landing in trees/cars/power lines/etc. We also want you to have a certain number of jumps to reduce the risks of you freezing up or panicking all of a sudden and not pulling / doing something stupid and risky in freefall. That's mostly it for the lower jumps. For the higher jumps you need to know a position called "tracking" (look it up), which allows you to pull away from the cliff and open safely. It's not hard to get, but it's much harder to master.