Lot of IIT-ians get through to IIM each year. Surely, this must be the game is rigged in favour of IIT-ians? The IIMs must surely have some mechanism to give IIT-ians an easy entry path?
The answer to both these questions is 'No'. Since there is no joy in taking a linear path towards answers, let us look at a comic strip first. This one from xkcd is fabulous.
Consider this - there is a group of n people who have cleared an exam arguably tougher than CAT. This group of n people also take CAT. Odds are that a number of people from within this group of 'n' people also do well in CAT. What we see here is a correlation, not causality.
If anything, it might be tougher for IIT-ians to get into the IIMs than the other way around.
Being one among the top 3% of IITians taking the exam is probably more difficult than being in the top 0.3 percent of people in the general category!
Quite a few of my IIM classmates were very smart people. And with absolutely no disrespect to them, I can say, on an average, the IIT classmates I had were smarter than the IIM classmates that I had.
To get straight to the point, this story of IIM being rigged in favour of IIT-ians is humbug. Very much like the story that it is difficult to get in if you are an engineer (More than 85% of entrants into IIM A, B or C are engineers). These are statements that people like to tell themselves in order to believe that the game is rigged against them.
The CAT paper does not give a huge advantage to engineers, the admission process is not built to favour IIT-ians, the overall selection process does not discriminate against engineers. So, whether you are an engineer or not, whether you are from IIT or not, there is a decent chance of proving your worth through this exam.
The preparation process and forums offer a great array of delusions to pick from; so that each person with can pick and choose one and believe that the game is rigged against them. Do not believe any stories that say merit gets bypassed in the selection process.
Notwithstanding the evidence presented by IIM graduates worldwide, the selection process, by and large, is merit-driven.