I played against Choate in sports, and am close enough in age we might have overlapped.
It is more competitive now. Expectations of ECs, in particular, are higher. There are also way more athletic recruits across all sports and more sports compete at a high level. When I went, sure hockey, lacrosse, sent kids to top programs but other sports, not so much beyond D3. We had recruits for hockey and a few other things, but not for everything. Athletes seem to take up a lot more slots, now. (Not to say they aren’t smart, but still need to fill those teams so all things being equal an advantage!)
There are also more kids doing way more math or science, younger. I don’t think we had more than 1-2 kids in my 4 years that went beyond BC Calc, and only a few did even BC as seniors. My kid goes to a top day school, and number of their classmates did BC calc in middle school, plenty were in Pre-Cal in 9th. My own HS is like this now too.
That said, other kids w/o incredible “wow” factors DO get in. One my kids classmates went to another school like Choate, and she was just interesting (and had good scores).
CSW and Putney are not in same league (in terms of competition) as Choate, for instance. They definitely turn down plenty of kids but pay a lot of attention to fit, IMO.