Logic behind masters programs admissions I'm 2 years out of undergrad and thinking about applying to some math graduate schools, specifically operations research and maybe financial engineering. I was a math major, with middling grades, and now I work in IT. 

My question is this: it seems that even for not top-tier schools, a high GPA is crucial, as are great letters of recommendation, wonderful GRE scores etc. Why? If I'm willing to shell out 50k for grad school, and I've met the prerequisites, and I show an interest in the subject, why wouldn't a school want to have me? Is it purely so that they can claim they have a low acceptance rate?

In other words, if a grad school (again, not top 10 let's say) has a surplus of people who want to take that program, in a field like math, why not just hire as many professors as it would take to teach them? 

I guess I don't get all this pressure to be perfect. 

Thanks! 

EDIT: I am referring to masters programs in particular, where stipends are not as common.

EDIT: Ok, Pete and Paul's answers make sense, given the current set up of grad school. I can imagine a different set up, however, that would make it less competitive for students, more profitable for professors and more about learning than prestige.
You should not pay tuition to attend grad school, excepting possibly certain "professional" schools like medicine, business, law.

That is, grad programs admit a number of students for which they have funding, and have faculty/mentoring support.

Programs do not get tuition money back directly (as counter-intuitive as this might seem), but have to calibrate grad admissions to match the faculty they have ... the numbers of which are determined by essentially unrelated issues. But, approximately, programs will not admit you if they are not confident that you can succeed in their program. So you should not begrugde that judgement. And, srsly, such judgements are made with much experience.

Yes, there is the issue of prediction of success, which is an element that did not enter in undergrad studies. That is, programs do not "admit everyone" and then allow people to fail. Nor is it just a matter of "trying hard". Beyond a certain point, there is an issue of natural ability. It is true that this is highly non-trivial to understand/detect/encourage. 

Yes, there is a considerable amount of disingenuousness in how we describe graduate programs...

Perhaps reaction-comments from the OP could help refine useful answers.