You are missing a key point. Quality of a business administration university stems not merely from the standards of the teachers and the style of teaching and other world class facilities provided, the biggest determinant is the quality of the students going in them in the first place.
Students from all around the world dream of making it to Harvard, MIT, LBS, Cambridge etc by scoring a high GMAT or related scores. The quality of teaching, faculty, teaching techniques, of most of the top notch universities are the same, however the end product of Harvard differs a lot as compared to let's say Texas university MBA.
This situation is more profound in Pakistan. Bahria uni in Islamabad may have better faculty and teachig style than IBA. However for students who get admission in both universities, the choice is simple. They will prefer to go to IBA as they are assured a job even before MBA completion. Most of the Bahria graduates may also get decent jobs, but not ALL of the students who pass out get good jobs. This is not the case with IBA.
And this make sense also. During your matriculation, inter, O and A levels, you have many students around you, some very bright & hardworking, getting 1st, 2nd 3rd positions consistently, and then there are others who can't get positions no matter how hard they try. IBA test is quite difficult than other admission tests. You will hardly see anyone who pass IBA admission test and fail to pass Bahria or CBM or LSE admission test. So the bright students have a better chance to make it to IBA.
No matter how much grooming (or lack thereof) they get during their stay in IBA due to the pathetic teachers quality, they will still, most likely be better when they go in the job market than others who gets admission in let's say Bahria or CBM or LSE, but could not pass IBA or LUMS tests.
The day the bright students will decide after getting admission in both IBA and Bahria or CBM, that they will decline IBA offer and will go to Bahria/CBM instead, only then the quality of students coming out of the institutions can be said to better.
Btw, I'm in job world and the performance of grads from different universities are easily distinguishable. There is a good reason why IBA and LUMS grads are preffered and given such a high salary as compared to others.