Howdy. The transponder is only needed in Class A (Above 18,000 ft. MSL), B or C airspace. It is used so Air Trafic Control (ATC) can see who you are, and how high you are, on RADAR. It is handy if flying near such air space, and necessary to enter (for the most part). Otherwise, not needed.
Since `coupes tend to fly away from such places, and transponders cost
money, they may not be in such as you see. Their "density" also
corresponds
with the amount of afore said A, B & C air space.
Percy, a genuine graduate of Air Traffic Control School, courtisy of your
friendly Air Force Recruiter
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