There are two main reasons for what you describe: very flat terrain in Netherlands and people living in multi-family buildings mostly. Thus people don't ride any substantial distance according to Netherland's own statistics [1] and don't physically exert while doing so.

In the US average commute is 42 miles daily, that's over 67 km, or more than two weeks of riding a Dutch 12-18 y.o. does, or a month of riding of a Dutch 35-50 y.o. I'd like to invite any Dutch, who believes it's the same in the US, to ride 67km daily for 5 days straight, even in their own flat neighborhood. It might enlighten them why cyclists elsewhere wear special clothes too! And this is without hills...

1. https://longreads.cbs.nl/the-netherlands-in-numbers-2022/how...