Its not about IIT, its about the field of Engineering. Most of the students in PCM in 10+2, pursue Engineering, and IITs, NITs are one of the best places for Engineering in India. Hence most of the Aspirants prepare for JEE Advanced as well as JEE Main Competitive Exams and aspire a seat in these prestigious institutes. Ofcourse, nowadays you do find many genuinely interested students who pursue Pure Science followed by BSc, MSc in Relevant Subjects and maybe a PhD, Career in Research, Teaching or Education. Many even nowadays pursue Non Stereotypical Career Options like Journalism, IPM Management or Law after PCM.