Dutchie here, same for me. In English it's easy to say 3pm or 9pm but in Dutch that would be 3 uur 's middags (in the afternoon) or 9 uur 's avonds (in the evening) so 15 uur and 21 uur is shorter to say. However, when it's "am" I always say 's nachts (at night) or 's ochtends (in the morning) to avoid confusion. But all digital clocks in NL are on 24h. I don't think I've seen anyone with a 12h notation on their phone or anything else. It's such a standard, I don't even think my oven and microwave have a 12h notation option.

I think it's just a case of uneducated ignorant Americans stuck in the past, while also having no clue there exists a rest of the world where people are not weird. Like with their imperial system and IALA buoys system (for the entire American continents by the way).

It is similar in Germany. Often with the word Uhr (like o'clock in english) added.

"3 Uhr" or "15 Uhr 30"

Same thing in Argentina. As usual, USA are the weird ones xD