Or that they're very complicated and take time to compute. Subspace is fantasy, so jump calculations can be arbitrarily hard.
I hypothesize that a jump solution is only valid at a certain time and place. So if you know you want to jump to Earth five minutes from now, and you're not moving at all, then that's just one calculation and you're good to go. But if you don't know when you want to go to Earth, even if you're still not moving, you have to continuously recalculate for the new conditions every minute. If you might want to go to Earth, Mars, or Jupiter, then that's three times more stuff to calculate, which takes up to three times as long. If you're also maneuvering in combat, then you don't know where you'll be or how fast you're going at the time of the jump, and that's exponentially more work.
Being physically at the entry point could also have something to do with it, because you could need sensor data about the subspace there or something ridiculous like that.