Streaming services are dependent on the distributors supplying them with movies. First thing to know about the movie industry and streaming is that the movie industry is conservative. Second that it is very possessive about its property.
This means that the movie distributors pretty much set the terms for the streaming companies and not in a way that is in tune with the times.
They dictate the time windows that movies will be available and often also the price at which it will be available to the consumer. Movie distributors often set these the same as for rentals of physical DVDs or VHS cassettes before that.
They also mandate the use of one of a few approved DRM schemes and other restrictions, and they are not so eager to allow downloads - and they could see large buffers as being that.
Next, you should know that online streaming is not cheap for the service. The servers and the networks cost real money.
That together with the remuneration to the movie distributors means that the profit margin can actually be quite small.
It has been a couple of years since I worked in the online movie streaming business, but I would be surprised if these things changed very much.