Of course possible major matters. If you aren’t prepared for econ or physics, of course they may choose someone who is. Same with having relevant ECs.
Of course, you can change majors. But the app is a snapshot of your readiness and thinking when you submit. It has to make sense. You may even need to explain this interest in a supp.
And they do try to balance the number of kids aiming for various majors. Why do you think they ask about possible major?
I believe Cornell is referring to declaring the major once on campus, spring of soph year. (?) Not freshman admission.