It's far less profitable than you'd think. Many of the competitors in the industry lose money.
Licensing fees/production costs/royalties eat a bunch of the money, but the infrastructure to run a streaming service isn't cheap. I know at least one of the competitors that people would call the most successful is paying a 9 figure AWS bill, plus whatever the costs are for all the caching setup that makes most popular content live in ISPs. Add the typical army of developers building apps for the mobile devices, the billing team, people doing recommendation engines, fraud detection/security team, tagging all content, and translating every blurb in a bunch of languages, possibly pay for creating subtitles for all of those languages... it's not cheap.
What is so frustrating about this is that so many of the costs would go down with consolidation. Every company has to handle the fact that age verification and privacy legislation in South Korea has to work in a just-so way, but every streaming service has to duplicate the logic. While the Netflix recommendation system probably can use some tweaks to take on all of D+'s content, I bet it'd be easier to tweak their system than to have every company have their own ML team handling recommendations. A lot of relatively low quality content wouldn't have to be made if we didn't need to make 'filler' to keep people spending sufficient time on a given streaming service while the next top quality release comes out.
In a happy world, competition brings more better content at a cheaper price for consumers, but when I look at the state of streaming, what we have is many competitors that aren't breaking even, in exchange for a streaming experience that keeps getting worse. And there's little chance it's going to get better until at least half of the worst competitors give up and go back to licensing their content to whatever the big 3 end up being.
And yes, we'd all be better off if we moved to the music model, where any subscription has 90% of the content, but do you really see, say, Apple, Netflix and Disney doing worldwide, full catalog cross-licensing deals?