Satellite links tend to be from directional antennas, owing to the bandwidth requirement and distance to the satellite.
They need a radome and a line of sight to the specific satellite of choice. Phased array antennas don't point mechanically, but they still need a line of sight.
Most ground-based communications can cope with a much less directional antenna, that can be located in a much less specific location, somewhere handy for power and geometry. Radar altimeters need a downward view, whereas GPS needs a hemispherical sky view.
Engines can be a source of stray reflections and their own emissions - there's a reason that the JSTARS aircraft still have the narrow-diameter low-power engines.