Well its highly un-common in pakistan. Other countries do get Visa's. I guess they dont want to entertain students with low budgets. They have a certain target for certain countries.
A good example here is .. a student visa on average in Pakistan takes about 8 months While in India almost Half or maybe 2 months sometimes.
So the equation is simple! Everyone knows where is point of interest
Anyway, Keeping this aside, the legal reason of them rejecting collage applicants is mostly Weak ties with home country, jobs prospects and sometime budget and Money (also known as $$)