It's hard from a caching perspective. Playing the segments you either need to explicitly define all the files you download in the manifest, which is inefficien

It's hard from a caching perspective. Playing the segments you either need to explicitly define all the files you download in the manifest, which is inefficient. Or you need to have a segment template which defines a formula for the segment names that increments with time. With that the player can generate the video segment that needs to be fetched for playback. When you create an ad break you'd have to have some backend which split the traffic routing so that media segments came from a different source. And that source would be unique for every user. This is hella complex for the CDN edge infrastructure at scale because you need systems which understand the state of the individual users session. It's much easier for them to manipulate the playback manifest so that it stitches different periods for ads and content. It's not impossible, just a PITA.