Well, the same reasons why we don't launch radioactive waste into space:
Rockets sometimes explode during launch
Septic waste is heavy - rockets no likey heavy payloads
Even when launched into space, stuff has a tendency to come back eventually
I'm pretty sure you're looking for more hilarious answers, though.
For a small scale concept visualization experiment, you could go build yourself a water rocket... and then use septic waste instead of water.