Based on their JEE ranks, 17-year olds get the chance to choose their majors. Interest in a subject area hardly plays a role, in most cases sole criteria is - which majors will help the most to have a financially rewarding career. In long-run we all are dead but in the medium-range most of the IITians end up working for the developed countries, especially the USA, either abroad or in India, currently in the IT industry.
I will cover the situation at IIT Bombay while I was there. IIT Bombay tended to be slightly more selective than other IITs because a disproportionate number of successful candidates came from Bombay region and preferred IIT Bombay over other IITs. Electrical engineering was the first to close its door around JEE rank of 200. Later, newly opened computer science department seems to have been even more selective. Mechanical and Chemical engineering were comparable and admitted candidats with JEE rank of around 500 or below. This list was followed by metallurgy, aeronautics and civil engineering. Physics, chemistry and mathematics were the last to go. After the rank of 1200 or so, the only choices Bombay region candidates had left for them were things like agriculture department at Kharagpur IIT. Most didn't take it. Possibility of being able to change major to a more desirable one was quite slim after that first chance.
In the USA, students are exposed to technology from childhood and have fairly good idea about what they want to do by the time they apply to colleges. While places like CalTech, Stanford are competitive, quite decent places like the state universities are open to most. There are other schools for students who don't go there. My guess is that almost everyone gets into a moajor of her liking even in medicine. Later students are able to transfer to other schools if they desire. Moreover, they can change their majors almost without restriction. Is this a bad thing? In my view no and is much better than IIT scene where most students go without much option.
IIT system, as it stands currently, is just an assembly line for producing workers for the developed world. Change it. I admit the reforms have to come starting from high schools where students are placed in science or arts stream as early as in ninth grade. However, IITs can made it somewhat easier for its students to follow the majors they like the most.