I believe it's because of performance load, at least for now. Because mirrors renders 3 more screens.
Unfortunatelly I can't confirm how is it with all at ULTRA settings, because my comp isn't strong enough to test it.
For interest, I noticed that trees and bushes has already been shown. It hasn't shown either those, several updates ago, if I remember well.
CPU Type - Intel(R) Core(TM) i5-4460 CPU @ 3.20GHz
GPU Type - Graphics Chipset AMD Radeon R9 280X 3,072 GB GDDR5
GPU Driver - Radeon Software Version 17.12.1
RAM - 8 GB
Win 7 64bit