Because they don’t know any better. They assume the watch is real, because as plenty of people have asked here, why fake a $4 watch?
If I buy a Rolex online that’s a fraction of the price of the real thing I know I’m getting a fake. If I buy a $4 Casio for $2 I can easily assume I’m just getting a bargain.
Why wouldn’t you? They’re the most popular watches in the world and very cheap and easy to mass produce. A decent fake of a luxury watch requires a decent degree of finish and a mechanical movement to look remotely passable as the real thing, but these are so easy that they often look almost identical.
Casio is far less litigious than luxury watch brands, so it’s easier to openly sell fakes compared to brands like Rolex which come down heavy on counterfeiters.
Casios actually feel very well and well finished made for the price. They’re known for making durable, quality consumer electronics. They’re cheap but they’re not trash.