Humans during this time followed game and ripening fruits. They had huge ranges compared to the neolithic. It's easy to imagine different bands frequently coming into brief contact and trading artwork or technology. If your band ranges over 200 miles in a year, it would only take a couple dozen bands to span from France to Japan across the eurasian steppe.

The art was also not likely limited to caves. Painting on hides, or exposed to the elements would not survive. Caves are ideal conditions for preservation.