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.
I remember driving through Iowa and seeing vast fields of corn and learning that the majority of that corn was not even destined for human consumption. That kinda blew my mind.
Plus is the fact that not all plants have the right amount of vitamins and minerals necessary to maintain the human body like meat does. Although it is possible, it does require research and monitoring to ensure that your getting all the nutrients you need. And yes, meat just tastes good.
If you're discussing complete proteins then all it takes is rice and beans. Not particularly difficult given that about half the world population survives on that without much meat.