Because they don't serve the purpose of a screen nearly as well as miscanthus. Which is to create a visual wall to screen animals from people you dont want seeing them or to keep screen your own movements from animals.
I've yet to see any native grasses that hold up to snow sufficiently to work as a screen. Most applicable shrubs are going to lose leaves during hunting season, take longer to be adequately thick to serve as a screen, require a larger area in order to not be seen through. On top of that, they are likely to become browse or bedding which is the opposite of what you want when the goal of the screen is to sneak in undetected. I'm currently working on using spruce, willows, and alders to try to screen off an area but if I knew miscanthus would work in this ground I'd plant it in a heartbeat.
If I got a good miscanthus screen around my field that has a road on 2 sides, the animals would actually feel safe to use it during daylight and not worry about all the traffic stopping to look at it or getting poached from a road.
I've never seen a report of miscanthus giganteus becoming invasive, if there is documentation of such please share. [Edit to add: the miscanthus i've seen sold for screening is sterile and only spreads a little through rhizomes. I understand concerns over fertile miscanthus becoming invasive]