The issue with lettuce is that it needs to be harvested, washed, transported and eaten within a certain time period. It can't be frozen or preserved.

Cattle can be transported alive, then when slaughtered, the meat can be frozen and transported.

Growing veggies IS a more efficient use of resources, but presents more challenges for a franchise such as McDonald's. This is why you see more veggies and salads on menus of independent food businesses.

I agree, especially on the shelf-life issue. Just-in-time manufacturing is hard all the way around, and even harder when it comes to food. Meat is much easier to preserve than produce in such a way that's acceptable to fast food consumers (especially when you're already eating THAT much salt!), meaning large chains like McDonald's wind up bringing more operations challenges in house as they add produce. This translates to a reduction of cost/revenue predictability, which at least initially might need to be mitigated by increased margins.

Having never worked at the kind of scale at which McDonalds operates it's hard to say whether or not these challenges are insurmountable. However my hunch is that if the demand was there, McDonalds would figure it out. They have a remarkable amount of resources that could be poured into these problems. Again, I'm as "red-blooded American" as they come when it comes to eating meat, but as a bleeding heart environmentalist I sure wish the market would change.