Because:

1. Wine contains sugar and aroma. The main food of ants is these smells. If the cup is not cleaned, it will usually attract ants.

2. If ants are attracted, it means that the quality of the wine is still good, at least there are few or no additives, it is natural.

3. Generally speaking, it is difficult to clean wine glasses completely. The glasses have pores that cannot be smelled by humans, but ants are different.