Arriving from a retail culture with late nights and seven-day trading, newcomers plan a shop for Sunday afternoon or after work and find the shutters down. Opening hours are now set municipally rather than nationally, which is why confident advice from someone living an hour away can be flatly wrong.
Historically it comes from the Winkeltijdenwet, the law regulating shop hours to protect small retailers and give workers evenings and Sundays off, and while it's been steadily liberalized, the cultural habit outlasted the strict rule. Most shops still close around 6 pm on weekdays, with one koopavond (late-shopping evening, usually Thursday) when things stay open until 9. Sunday and Monday-morning hours are now set municipally rather than nationally, which is why the experience varies so much by where you live: big cities have mostly adopted koopzondag (Sunday shopping) as standard, while smaller towns — especially in the Bible Belt running through Zeeland, the Veluwe, and parts of Overijssel — still close entirely on Sundays and sometimes Monday mornings too, out of church-going tradition rather than law. Supermarkets are the exception almost everywhere, staying open into the evening and increasingly on Sundays. If you've just moved from a 24/7 retail culture, the adjustment is less "everything is closed" and more "check the specific city," since Rotterdam and a rural Zeeland village are basically operating on different rulebooks.