This isn't universally true - some meat is grown by grazing "free" plant calories. But it is true for grain fed cattle -- for most US industrial meat production.
The methane production issue is in addition to the plant/meat production inefficiencies.
Why is this a problem? Animals also produce higher quality protein with higher bioavailability, have a better amino acid profile that better suits what humans require, and provide nutrients that people cannot get from plants.
That's on top of the fact that a lot of those plant calories aren't even consumable by humans.
It's a problem because of the aforementioned disproportionate energy consumption.
> "Animals also produce higher quality protein with higher bioavailability"
Hey look, you don't have to pitch me on meat -- I'm basically a carnivore. Personal preferences don't change the underlying engineering dynamics.
> "and provide nutrients that people cannot get from plants."
I love meat and I'll defend it as stridently as anyone, but this just isn't a rational argument. I want meat in my diet, but I do not require it to live. There are many ways to achieve a balanced diet either with other animal products, or without meat altogether.
The really weird thing about diet is that many (most?) of calories a typical American consumes are in excess of a healthy amount and are hurting us rather than helping us. It's a very strange dynamic, because the vast majority of Americans suffer from excess nutrition -- not malnutrition.
These excesses not only hurt our health, they harm the environment. Fat people create far more greenhouse gasses per capita than thin people. I eat meat, but I also eat healthy portions -- which puts me far ahead of most people regardless of their diet in terms of the resulting carbon footprint.
You need to produce much more plants to feed the animal first instead of directly feeding you. So to stay on topic (unlike your "quality") this process uses more fossil fuels.
Nutritional yeast is tasty and naturally has b12, and most cereals/grains are fortified with it.
> amino acids [...] only found in meat
This just isn't true, you can get all amino acids from plant-based sources like soybeans, lentils, and peanuts.
Source: I have been vegan for 5 years and I still have a healthy 6 minute mile, 25 consecutive pullups, V5-V7 range boulder problem climber, etc.
As to metabolic equivalency, that is a very good question, studies so far indicate the accessibility of amino acids in plants to the human metabolic pathways varies within our species somewhat widely.
Supplements are an easy solution, but I think what you're missing is that you don't necessarily need the massive amounts of those nutrients that is reflected by the meat industry. We certainly do not need to eat as much meat as the meat lobby wants us to eat. In fact, even by your nutrient argument, we need hardly any meat. In fact, of all the things we eat, the most amount of meat we'd need to eat, if we needed it at all, would be the smallest quantity amoung all those things. And really we don't necessarily need meat at all. Using a nutrient argument to support the eating of meat is a bad argument.
In other words, fertilising the soil with fossil fuel derived products is just.. more profitable.
The solution is probably carbon pricing of some kind for fertilisers: the cost of organic farming needs to be roughly the same. Agricultural subsidies are already enormous, so some of them could be directed to food prices directly, to ease the transition for consumers.
