Since some people argue that being vegetarian is more expensive than a standard diet, why is meat so expensive?
Meat costs what it costs. The arguments of ‘some people' do not impact the price of meat (other than to create more demand which pushes up prices, but the effect is minimal).
A better question would be ‘Given that meat is expensive, why do some people argue that a vegetarian diet is more expensive than one that includes meat?"
But ultimately people will argue for or against just about anything, usually in an attempt to validate their own worldview. People who like meat can compare low grade ground meats to organic pre-packaged foods from boutique grocery stores. People who don't like meat can compare beans and rice to filet mignon. Whatever arguments they come up with will not have a significant influence on the price of meat.