There are actually things to test in the integration between the rocket and satellite. It's not like the satellite is a dump piece of steel sitting on top of the rocket. The payload has its own computer, power systems, and propulsion. The interactions between that and the launching rocket / ground systems matter. Also mating the two is non-trivial, so you can push the static fire back closer to the launch if you are doing it with the payload on board.

A static fire without the payload on the rocket was already done in Texas several weeks ago. This is the final dress rehearsal before launch. I always thought of these tests as low risk, but I guess nothing in rocketry is low risk.