There's no guarantee that a sensor inside the airframe is really sensing the true static air pressure. You'd have to prove that (for certification purposes). That wouldn't be trivial.
Even if in steady state the pressure were to equalize, during rapid climbs or descents, it will take time for the static pressure inside to catch up with changes outside. that means the altitude and airspeed would misread.
This effect is even seen with existing sensors - an aircraft with a trailing cone (for experimental purposes) has to be very mindful of this during dive testing. And that's with much less volume to equalize than a large chunk of aircraft volume.