Why can't we operate at high frequencies (>0.5GHz) when having a resonant inductive coupling circuit?
I assume you are referring to WPT standards  in 4,8,11,22 kW levels or small power for mobiles. 


stray energy damage to humans increases with frequency measured in SARs mW/g  this must be absorbed by careful lossy ferrite .
maximal energy transfer is when impedances are matched and this is far easier with lower f due to stray geometry of capacitance nad wire inductance.
the number of coils is an attempt to match impedances for mismatch in position orientation and reduce the max. Q in any stage which affects resulting tolerance errors
the mutual coupling M falls off sharply as the gap starts to exceed the diameter.
THus lower frequency lends itself to greater range and lower mutual coupling loss.


However there are people who believe Wifi ROuters with rapid directional beam antennae can be used for WPT at GHz range , but I think health risks are high the eyes. (caterogenic)