The simple answer is cats by nature crave fresh tender eatable vegetation.
Outdoor cats can satisfy their desire for fresh vegetation.
But, when cats are kept indoors all the time, they have no opportunity to eat fresh young grass as nature intended. So they chew on and try to eat our houseplants. If you remove the houseplants - or say you just don't have houseplants, we found our cat's tried to get outside to satisfy their desire for eatable vegetation. When the oat grass gets about 3 or 4-inchs high, put it near your cats' dry food or water bowl. No more eating the houseplants - No more trying to get out of the house to satisfy their desire for fresh young tender blades of grass.