@VD15
Meme answer: they lazy
Serious answer: it's likely that to safely make the transition from downside-down, to upside-down, and then back to downside-down driving, in a single straight, would take much too long, as it would need to be done in a safe and steady manner. Any track specifically made for this would simply be bad and boring for racing on.
Also, no matter how much downforce the car produces, there are still serious risks associated with driving upside-down, that would make FIA think your legally insane just for proposing it.