Low subscription prices are sustainable when the subscriber base is growing and continually bringing in more money, but at some point most of the customer base has been reached and then the only way to increase revenue is to raise prices. Until recently, streaming customers have had their subscription prices subsidized by subscriber growth.
Why can't streaming companies be happy with the revenue they already have? They need to increase revenue not solely because shareholders demand it, but also because the price of content continues to increase, especially when content providers know they have leverage when licensing terms are negotiated. The bigger a streaming company gets, the more eyeballs will see the content, so content providers feel justified in asking for more money. Also, streaming companies generally have libraries of content that increase in size over time, so the total money they pay for licensing will also continue to increase even if the price of each piece of content does not. On top of all that, they need to pay for the infrastructure to deliver all the bytes.
(This is why Disney's streaming service is in such a good spot -- they already own the content, and they have a huge back catalog.)
Say what you will about the cable companies (and they really do suck) but their pricing was not entirely a result of monopolistic practices. Price pressure from content owners was also a major factor, and it is such a strong force that streaming companies can't escape it.