I should have said play/train year round. There's no doubt there's still kids who play multiple sports - but it's indisputable that there are less kids doing it nowadays, especially at big schools. Just google high school sports specialization.
Also - I was talking about 3 sports - there's a huge difference between playing 2 sports and 3. Sure a bunch of football players play football and wrestle - now add trying out for baseball or lacrosse - against kids who play/train year round. The odds against them making that team are much lower (esp at large schools). When you say you've never met a kid who just played football or basketball that is not my experience, or the experience of other people I know who have kids that play those sports.
At big high schools around here (and I'm sure other places) - they make a big deal with getting the kids together who sign to play a sport in college. At my kid's school there's usually 10-20 kids. I can say with certainty that in four years my kids were there (going back a few years), there was not a single athlete that signed to play a sport in college that played three sports in high school. By far the majority only played one. My kid's school was extremely competitive (they routinely win 4-5 state championships a year), but the same was true of the other big schools in the area (I knew many of those athletes from youth leagues, travel leagues etc). Of all my neices and nephews who played sports, and kids of friends around the country - I don't know of a single one that played three sports. I'm not saying that there aren't still kids who do - I'm saying it is much more rare than it used to be, especially at large schools. Many kids/parents think they need to start specializing in a single sport early on to play in college, or honestly to even make their high school team at many large schools (beacuse so many other kids are doing the same).