Because when people sign up kids for sports, the thing they want them to learn the most is competitiveness and hard training. Everything else is basically perceived as being lazy. As a culture, we tend to turn evrything into grand moralization and duty.

So, consequently, sports club that is not trying to win is seen as lazy and bunch of loosers. And quite openly.

I think there's a big gap between "trying to win" and aspirations to be an actual professional or Olympian.

As someone who was quite competitive in several sports growing up, I don't think "not aspiring to be professional/Olympian" was frowned upon at all.

If it is today, that is extremely pathological.