A berg could just have meant a hill. This is not limited to natural hills. Artificially elevated land could just as easily have a name ending in -berg. Add to that the many farms, villages and cities throughout Flanders and the Netherlands with the same suffix or a variation thereof and you get many occurances of this toponymic surname.

This is similar to a lot of previous questions, and it seems a common myth about specifically the weird Dutch ones is that they were a form of protest against Napoleons demand for everyone to have a surname. In most mentions on this subreddit however, it's specifically mentioned as a myth, although I can't find any mention of how it actually is.