There's a handful of reasons that happens, but one of the biggest is that those ads are placed by a variety of different providers. More often than not when a large buyer goes to place a buy (think on the order of P&G promoting a new product, the snack conglomerates, movie studios etc.,) they'll place an order based on a certain number of impressions. they'll also mandate a certain viewability threshold and a number of other conditions.

In order to execute that buy, the media desk has to make purchases from a variety of platforms. Those platforms also buy and resell inventory from a mix of other places. Segments aren't very well defined, so you end up competing against yourself to buy the same inventory over again.

FWIW, a lot of those OTS services also have very traditional media desks. (e.g., https://nccmedia.com/)