It is a question of frequency reuse and the difference in population densities.
The radio frequency spectrum is a limited natural recourse and thus as many communication links should be able to use the same frequency. On HF (especially in the 10-20 MHz) range, the reuse factor can be as low as 3 due to the ionospheric propagation (i.e. there can be only 3 independent communication links at a specific frequency globally at similar power levels). On VHF/UHF/SHF the communication range is mainly limited due to geometry to distances of 30-100 km, thus, the global reuse factor is significantly larger.
By limiting the transmitter power, the reuse factor can be much larger, i.e. more devices will fit into the same frequency range.
The free space loss is inversely proportional to the square of distance, while in urban environment, the received power is inversely proportional to the 4th power of distance. Thus, with transmitters and receivers well below the roof line, the propagation distance is quite limited and a large number of SRDs can be used on a single frequency.
In Europe, the population density is high in most of Central Europe, while in the USA, the population densities outside New England and some metropolitan areas is quite low. Thus, a lower frequency reuse would make sense in most of USA (i.e. allowing higher power levels).
I do not want to defend traditional channel exclusive allocation systems, since a frequency range is unused most of the time. It makes much more sense to use some kind of ALE (Automatic Link Establishment) or some frequency agile system (like CDMA) to get the message trough.
I do not know, if the row about using unused UHF TV channels in the USA for other communication has bee solved, but this at least shows the problems of packing as much as possible communication into a limited frequency range.
IMO, radio communication should be used only when it is absolutely necessary (e.g. moving platforms), but as soon as the signal is within a fixed wire/fiber network, the signal should be transferred to the wire, freeing up the radio frequency for other users.