To expand their geographic diversity, and to accept very strong students.
But really…a lot of schools accept international students because they will be paying the full cost to attend…or close to it. Or they only accept the limited number for whom they are willing to give need based aid.
If you are from a very underrepresented country, your chances of admission might be better at some of these places than if you are from say…India…where these elite schools get tons of applicants. Your application will be reviewed with other international students from your region, and some regions have very very strong applicants…and these schools accept 5% or so of them.