If they had planned ahead, they could have had a system to lift a bunch of satellites off together, then take off/replace the bad one, and then put all the rest back again.
My guess is that they did a cost benefit analysis of doing this, and realized it was worth more to them to launch a dead satellite and verify it deorbits as planned than it was to miss a launch deadline, take apart the stack, fix the bum satellite, reintegrate the stack, put it back on the rocket, and schedule a new launch date.
The satellite's cheap - that's literally the point. Verifying failure modes with NASA and the USAF? Now that's expensive. It's worth more as a science experiment than a time sink.
There could be some systematic failure that affects them all, and just this was the first to fail. If the rest fail over the next 2 weeks, that will be a big goof.
PS: besides, who said they don't know the reason?
SpaceX is going for vast scale with this project. Failures up to and including the loss of a launch vehicle can't derail the rest of it.