General Idea about Ants:
Ants can eat a wide variety of foods. They eat plants, other insects and sometimes steal food from other insects. They also like sweets, which explains why they're more than happy to invade a picnic that showed up in their area.
This ant-stealing-our-picnic has its comic aspect (as shown in movies/tv shows) which is a reason behind its popularity. In many shows, you probably see that leading character(s) goes/go to ant's area (or nearby area) and they steal foods and carry these to their home. This scene is depicted in a comedy way to make viewer laugh and it worked out.
Stealing food might be their way of saying that you decided to have a picnic on our turf and now you will pay.
In some species, there is a forager who finds foods when finished. When food is laid out on a picnic, smell of nice food, which is of course, makes them come out and steal it.