We can't in some countries because the 27 and 75 Mhz is allocated to other uses.
With the EEC the members agreed on using RC equipement on the same ranges, namely 40Mhz for surface and 35 Mhz for air.
If this was enforced correctly a sanctioned race would only allow 40Mhz but we see guys with radios bought in the US using 75 and even 27 (this is really risky). They run the risk of getting signals from wquipement other than RC. I stick with the approved ranges.
(I wonder if I can use 40 Mhz in a plane unable to take off
) LOL. That is surface anyway....