The exact percentage is difficult to tell because the data is available only with the Training and Placement Cell of IIT Madras. The biggest problem with Civil Engineering is that you will not get any immediate opportunities in the same stream. So many of the students who need a job immediately with a good pay package usually shift to other streams.
It is not the case that people are not interested in pursuing core civil engineering jobs. But they want to study what can give them jobs immediately. Unfortunately, this is difficult in core civil engineering jobs. These jobs demand higher degrees which the undergraduate students lack and thus are not considered.