Because not all colleges are equal. This is a prestigious technical institute, not a parliament where your argument of perfect representation would make sense. What happens when the people who get in on quota can't cope with the rigour of the course and drop out? Is that not wasting a precious seat? With a batch that is not entirely selected on merit, the quality of the institute will go down...

Reservation is a good idea up to a point, say 12th standard. But some things need to be merit only. Would you like a hospital system where the doctors are not the best of the applicant pool, but were selected on the basis of caste? Or an army like that? Quality matters...

And there are other ways of helping them out. Maybe hike the general category fees to make quota guy's tuition/books/exam fee free. But not bending the entry barrier in their benefit.

Besides, soon it will be 70 years since independence. In some more time 100 years. When will this system be rolled back? You cannot expect to keep this on forever, because that implies that you believe that sc/st/obc people are inherently weaker and will need a crutch for the rest of their existence. Is this not against the idea of reservation, that it was temporary and only meant to bring the backward people up to speed, not give them undue advantage?