Fake Plastic Trees
Plants are messy. You have to keep feeding them, they get sick, they die.
Plastic foliage has none of these problems. They stay perfect and remain so for hundreds of years (even after humans are gone).
Maybe we put them up as placeholders - things which we hope nobody pays too much attention to. We want the advantage of the imagery, without the effort that goes into it.
The problem is that when people do pay attention, it has the opposite effect.