Two factors: asymmetry and noise.
In a great many applications, there is one common transmitter, serving many small receivers. The receivers must be compact and cheap. The transmitter must be static, and usually high powered in order to cover a commercially viable area with sufficient power to overcome background noise.
Electrically small antennas have low radiation resistance: on the one hand, the received signal is very weak; on the other, the noise due to the antenna's own resistance is small, so the signal you do receive, though weak, is still largely received signals, and atmospheric and ambient noise. Low noise amplifiers aren't terribly hard to make (a few modestly sized JFETs will do fine for AM/FM BCB purposes, or fancier kinds in the GHz), so again the detected signal is largely from the airwaves rather than antenna and front-end noise.
There are also some special cases that kind of flip this around, but not in such a way that symmetry results. Take deep space probes for instance: by necessity, they are equipped with a fair sized (low ~meters) antenna, and not very much power (tens of watts). The ground station, on the other hand, is made up of a worldwide network of massive radiotelescopes (10s of meters or more), synchronized so that gain by synthetic aperture is possible. (Hmm, honestly I don't know if they go quite that far for the DSN; it's possible, as it's done regularly for VLB astronomy.) Bidirectional communication is necessary, often with greater bandwidth requirements transmitting from the probe (since it's the thing collecting data; I don't know how large the uplink payloads usually are, say for command or software updates if applicable, but I've got to imagine they just aren't as much overall?). But as it's rather impractical to fly a 25m+ antenna to space, of course this is the compromise made. Not that it's much of a compromise, anyway: on the physical level, antennas are necessarily symmetrical, no matter who's transmitting or receiving, and the total link gain is the product of both antenna gains, period.
Tim