Why? Moral vegetarians is a quite different topic, and there is an answer for this too. Current rates of population growth are unsustainable without animal food sources. So, the price would be lives of children and weak adults of poorest communities.
Most people I know who are moral vegetarians do not demand that the rest of the world be as well. They simply object to eating the flesh of animals themselves, or (for the vegans) eating any products derived from animals which did not consent.
You do also realize that another solution for unsustainable levels of population growth is to curb population growth, by better access to and encouragement to use birth control, right?
These topics are subtle and the current fashions among some social classes in some regions of the world cannot be considered as something more than mere fashions.
For example, that "obvious and easily observable health benefits of being vegan" are related to the avoidance of processed foods and heavily unbalanced diet, rather than avoidance of "animal flesh" or dairy products themselves. This is just the confirmation bias and similar factors at play.
Those, who have studied human physiology, not just cellular metabolism, which is also important, could tell that the problem is too complex to be dealt with by silly slogans like "avoid red meat". There are many subtle ratios involved, not just an ability of a particular enzymes to convert a particular molecular structure into molecules involved in the metabolism and support of the homeostasis, but the ratios among these processes. The balance is what is important. In other words, "substituting the protein sources", is not that simple as it seems.
There is no scientifically proven reasons to avoid dairy products, except for individual cases of lactose intolerance, while there is a consensus that the food industry is the cause of the problem to which veganism seemed to be a solution.
Tribal eating habits is a quite interesting topic, btw.
As for birth control, this is quite another topic, which, perhaps, cannot be discussed in this context. For some members of so-called "urban-middle-class" in some region of the world the policies of forced birth control are acceptable, while in other societies there are quite different reasons about how many children should a family have and why. And, of course, it is not for some "vegans" to decide what other people in other regions of the world should do.
Source? I believe that animal food sources are a vastly less efficient use of land and resources than plant food sources--the whole "20 pounds of grain to produce one pound of beef" thing. Certainly nutrients can be in short supply in a meat-free diet, but it's easy enough to get them from vegetable sources and add supplements.
