The relevant regulation (for FAA Part 25 at least) is "§25.1325 Static pressure systems"
Incidentally, this pretty much answers the original question - why are external ports used - as the reg explicitly requires external ports:
(a) Each instrument with static air case connections must be vented to the outside atmosphere through an appropriate piping system.
The regulation also requires that the system be designed, as CJD says, to minimize interference. The reg also requires that the installation be proven against icing, to the point about susceptibility to icing.
In terms of HOW this is done in cert, the basic approach is to compare the sensed pressures from the production air data sensors to a known reference. This reference is typically provided by a trailing cone which allows for a static pressure sensor a significant distance behind the aircraft, where the influence of the aircraft on static pressure is assessed as minimal. The aircraft is flown in a range of flight conditions and configurations, and the differtences between the reference trailing cone pressure and the production system pressures are termed the PEs or Position Errors.
A set of corrections are then devised, called the Position Error Corrections (PECs) or Static Source Error Corrections (SSECs) which may be supplied to crew as a set of correction charts, or directly implemented into the systems as an electronic correction before the data is presented to crews.
For systems where an electronic correction is not possible, physical changes can sometimes be made to the installation (changing the location or detailed shape of the sensor, or of the fuselage near the sensor) to aerodynamically correct the pressure value.