While yes, there are jet packs today, they are all extremely difficult to control, very easy to get yourself killed in, fairly easy to break, and most certainly not compact. In addition, the fact that you have highly compressed fuel on your back means that a stray round (or a well-placed flamethrower spell) will turn you into a bomb. They haven't yet been able to make a practical jet pack that could stand up to any kind of abuse, certainly not the kinds of abuse you'd see in Shadowrun.
Remember, folks, the phone book can be applied to DMs as well.