Man, 10 years ago, Netflix used to actually be good.
You do realize that Netflix only introduced their streaming service in 2007 (9 years ago), right? Before then, you would manage a list of DVD titles and Netflix would mail them to you as they became available. They still offer this service if you'd prefer DVDs to streaming.
The difference here is that nobody needs to get permission to rent DVDs. If you go to your local Walmart and buy every DVD on the shelf, you could then rent them to anyone you want for any price they're willing to pay. The media companies can't stop you from doing this. So Netflix was able to rent any DVD that they could buy. They did enter into some deals to get DVDs at reduced prices in exchange for some concessions (delaying when the DVD would be available to rent), but these were terms that Netflix opted into. The could just as easily have told the media companies "no" and rented DVDs anyway.
When they moved to the realm of streaming, though, Netflix needed to get permission from the media companies for the rights to stream content. They can't just take a DVD/BluRay, rip it, and put it online. Not if they don't want to face multiple lawsuits. Instead, they need to ink deals and many media companies are withholding content because they see "Internet = piracy = we lose money." What they don't realize is that making the content available via Netflix (and similar services) is they best defense against piracy. (Why pirate Random Great Movie if you can just view it on Netflix instead?)
Netflix would put all of the world's TV shows and movies on their service if they could. They just can't afford to pay every media company for every show so they have to make deals where they can and figure out how to manage their costs versus content offered. It's a much different game than their DVDs-by-mail of 10 years ago.