To play devil's advocate a bit: running software isn't free. If you're paying for an online service, the hosting cost to the company does scale per-seat. It's easy to say that the company only paid the cost of development once (and even then, more users = more development demand), but you're not a user of a single unit of developed software.
For streaming services, I think the development-hosting needle swings way towards hosting. Streaming video is the vast majority of their costs and efforts, so scaling prices per-user is the only sensible option. The "software" of those services is bad and interchangeable, and not what anyone wants or is paying for in the first place.