why does microwave communication need line of sight?

I went to a TV station recently and there, they told microwave communication requires line of sight. I have read about line of sight, but can anyone tell me why line of sight is required for microwave communication?

Review different mechanisms of radio wave propagation Radio propagation - Wikipedia, the free encyclopedia

Because you want your signal reach the reciever in the shortest possible way without getting diffracted, refracted,reflected or absorbed by the atmosphere or any other material. its that simple.

at lower frequencies less than 2 MHz,waves travel as ground waves however at higher frequencies any obstruction between receiving and transmitting antenna will block the signal.

Regards
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Because microwave freq is always higher than mobile telecomm.
At microwave, the atmosphere attenuation is very high of the link. And the link budget should have about 20-40dB redundancy for rain. So selecting line of sight could give more redundancy.