Why microwave links are in the higher microwave spectrum (I don't know exactly but I guess it's around 20-40GHz). If by Friis Equation higher frequencies have lower range then why use higher frequency rather than low frequency?
Microwave links require directionality or high gain antennas and they mainly use dishes (parabolic antennas) at each end. The high directionality means that a decent amount of the energy sent by the transmitter is picked up by the receiver. A higher operating frequency means more directionality for a given dish size: -
Notice the lambda term in the denominator; as frequency rises lambda falls and gain increases. The term D is the dish diameter. See this web page for more detail.
If by Friis Equation higher frequencies have lower range then why use higher frequency rather than low frequency?
First of all let's clear this misconception up; a transmitted radio wave has exactly the same "range" in free space irrespective of the frequency i.e. it reduces in power density as distance squares. But, of course, for a simple antenna such as a quarter wave monopole, its optimum length decreases with frequency hence the power it can extract from "the ether" also reduces. You have to start thinking about a receive antenna behaving like a fishing net. The wider the net, the more received energy (and fish) it can collect.
The Friis transmission equation has a lambda squared term in the numerator (causing link loss to increase with frequency) but, this effect is cancelled by the dish's lambda squared term in the denominator.
Friis link loss equation: -