Oh look, it's my wheelhouse!
I've worked on a number of different ebook and video products for distance learning and higher education (I have a pretty good guess as to which one you're using). I'm somewhat limited in the amount of detail I can give here but I can say that:
It has nothing to do with technical limitations. Streaming video is basically a solved problem these days; everyone's using one of several whitelabel services for hosting and streaming, and offline video has no technical issues with seeking to arbitrary points in the stream.
In education it has nothing to do with forced viewing of advertisements (though that of course is a factor in non-education contexts).
The reasoning behind including this "functionality", instead, is pedagogical -- the concern boils down to the fear that students will just skim ahead in the video and not learn anything. I personally believe it's totally misguided (and somewhat condescending) reasoning, and creates a number of usability problems, but, well, you can't win them all.
(A less frequently verbalized but still real factor is that video is relatively expensive to create, so publishers feel an incentive to make sure the users actually sit through the stuff they spent all that money on...)
One exception to the above: A handful of products have quiz questions interspersed within the video, some of which support branching to different content depending on the student responses -- in those cases there is a good reason to prevent the student from skipping past those points at least. This can be challenging to implement properly , so unfortunately tends to get lazily designed as "just don't let them skip ahead at all".