Probably because the beyond/impossible offerings sell more and they're only willing to put a limited number of vegetarian items on the menu so the others are crowded out.
I’ve never understood the “pretend meat” category. Lots of vegetarian food tastes amazing and is good for you. It doesn’t have to pretend.
It’s like over-boiling broccoli and covering it in cheese sauce: easier for broccoli-haters to eat, but totally missing the point.