1. There are some drone project
2. Drones aren't easy. In theory they are simple but in practice they are a lot more complicated
Many drones will use a 'simpler' microcontroller rather than a complex SoC like the Pi. For example the ArduPilot controller uses ATMega microcontrollers. Advantage is that they require less power and will 'boot' quicker than a Pi with a full OS will.
It is certainly possible to create a drone with a Pi (since it has been done) but you'll need a bigger battery for similar flight times.