Because we use the land that could be used to grow enough food to feed many people to grow food for cows, which then feed fewer people. By buying into this system, you’re propagating inefficiency.

To add to idiomaddict's great points, the animals don't eat exclusively grass. In Australia (assuming based on your instance): "the latest estimate (2017-18) of annual feed use in Australia is 13.58 million tonnes" (SFMCA).

This includes "cereal grains, legume grains, vegetable protein meals, animal protein meals, cereal milling co-products, minerals and vitamins" as per that same source.

I often see people use the deforestation of the Amazon for soy crops as a sort of gotcha for vegans, even though most soybeans are grown for use as animal feed (in the Amazon, mainly cattle). Incidentally, cattle farms are also responsible for much more deforestation in the Amazon than soybeans, but I digress.

I'll also note that grass-fed beef has often been shown to be as bad (or sometimes worse) for the environment than feedlot beef. It also can't scale to meet current meat consumption.

Oats, barley, wheat, rye, rice and bamboo are all grasses humans can eat.