A full answer to this would take volumes.
Flying an airplane according to Instrument Flight Rules (IFR) requires a qualified pilot & airplane with duplicate radio and Nav. equipment and an airframe that can handle all the environmental conditions (including rain, ice etc). There are multiple instrument ratings requiring particular equipment and qualifications and allow operations in different visibilities and conditions. The pilots has to have completed and been tested under a long list of conditions and have extensive knowledge of meteorology, navigation, equipment & avionics, theory of flight, airmanship, human factors (illusions etc) and safety. Take a glance at pilots basic bible called From the Ground Up to understand that flying in instrument conditions is in a different league to operating a small drone.
On top of all this, remember that airplanes under IFR operate in restricted areas like approach paths into airports, tracks in the sky and predetermined flight levels following radio navigation aids.
all this for the safety of the pilot, their passengers and life on the ground.