The basic problem is that to get 1000 calories of beef, you need to feed the cow something like 10,000 calories. So growing a cow is actually growing an entire field of wheat/corn/etc., then feeding it to the cow, then eating the cow.
Farming all of those crops for the animals takes up a lot of land, consumes fresh water, produces wastes, and uses oil/gas (for farm equipment directly, or to produce things like nitrogen fertilizers) which produces co2. Cows also produce methane (that’s the fart thing) which is a bad greenhouse gas.
You could just eat the wheat/corn/etc. directly (most of the time) and skip the meat step therefore saving a massive amount of environmental impact.