Especially when there's no fucking reason to fuel up while crew are aboard.
Actually there are a number of reasons to fuel right before launch, some specific to SpaceX/Falcon 9 and some in practice.
1) If you fuel the rocket you have a lot of people that are working in and around the rocket when it's in a hazardous state for a long period of time, with no means of escape. Think of all the technicians in the white-room who are strapping the astronauts into the capsule etc... The astronauts, when strapped into the capsule, have a good escape system that will get them away from the fireball.
2) In the old days, it did take hours to load propellant into the rocket, and having your astronauts strapped into their seats that long was lunacy. With the Falcon 9, that process is down to 40 minutes.
3) The Falcon 9 requires this. The design as it stands depends on sub-chilled propellants to achieve the required performance. This means that the rocket can't sit for long on the pad fully loaded, certainly not long enough to strap in the astronauts.
All in all, with the way that the Falcon 9 works, and the reliability of the launch escape system, it's actually safer to load the propellants when the astronauts are already packed in.