Why microstripline is compulsory to generate high frequency signal in the range of 2 Ghz or more
Well you really want some sort of controlled impedance, and Microstrip or Stripline based design (different things!) is the way to get there on a PCB. 

You could use coax cables soldered in place, or even (rather large, but it is done) waveguide based systems, you usually see this one in things like radars and plasma heating systems where the peak power is high enough to make burning the substrate or arcing a problem. 

As the frequency rises the length of a connection over which you can consider both ends to be equivalent falls (Generally about 1/8th of a wavelength at the highest frequency of interest), so by the time you hit UHF or low microwave the traces on the PCB start to act as components in their own right.