For example Oxford and Cambridge have seemingly very high acceptance rates... but this is because in the UK students can only apply to one top-tier university. The Ivy League has tiny acceptance rates because many students apply to many of them. I guess you could apply to literally all of them if you wanted to? Does that make them harder to get into?
It is indeed common for one person to apply to most of the Ivy's, and schools care about yield so there are also various tricks employed like waitlisting students they think are not seriously considering the school (Harvard doesn't really do this though, moreso a thing with the tier 1b schools).
However there is the flip side that it's rare to apply to one of these schools as a random "why the hell not" reach. Probably 90% of the applicant pool has a good enough background to feel they are a serious competitor, which would be the top fraction of college bound seniors overall. I would guess the IIT exam is a less self selected pool.
For example Oxford and Cambridge have seemingly very high acceptance rates... but this is because in the UK students can only apply to one top-tier university. The Ivy League has tiny acceptance rates because many students apply to many of them. I guess you could apply to literally all of them if you wanted to? Does that make them harder to get into?