Think grocery stores.
Much as content producers may want their own self-focused delivery stores, there isn't a chain of Nabisco Stores or such around. Some stores kinda get there, with heavy promotion of "store brands" or label-focused ones like Trader Joe's and Aldi, but while they win on price their success is tempered by lack of range. Likewise, Dreamworks and Sony and Disney etc. may want to provide direct sourcing of their material to customers, and may succeed to some degree, what most customers want is a centralized "I don't care who made it, I just want it" aggregator & distributor and not have to hunt down "awright, what studio made Shrek...good graphics, so it must be Pixar, which is owned by Disney, check Disney.com...nope, not there...Sony Animation Stuido? they did Open Season which looked similar, check Sony.com...nope, aw crap ya wanna watch my Over The Hedge DVD again?"
So the aggregator/distributor model is key. Publix, Wegmans, Piggly Wiggly, IGA, Whole Foods, etc. make deals with major product/content producers for distribution rights. Smaller ones have to pay for those rights, bigger ones are paid; the supply-and-demand model works out in its intricate complexity. Different distributors provide one benefit or another (ditto deficiencies), which when in competition with other near-identical distributors means innovating and maintaining something which garners more attention. They can exist side-by-side, what with Super Walmart carrying the greatest quantity & variety of most popular stuff cheap, Aldi giving near no variety (but pretty good what's there) stuff dirt cheap, IGA being "it's a grocery store, no more or less", Wegmans selling everything imaginable albeit higher priced, Whole Foods having high-priced premium selection, etc. Likewise, Netflix gets first-there broad-spectrum appeal, Hulu having better TV options, ABC/CBS/NBC/etc. having super-popular but limited offerings, Redbox (assuming they go streaming) featuring broad-range latest big-name short-availability content, etc.
The content producers can't develop enough following to garner customer loyalty to a single streaming source; nobody is a Dreamworks fan to the exclusion of most everything else. An aggregator/distributor can, however, will have to license Dreamworks content because customers want it (even if they don't know who made it). No aggregator can satisfy all customers; improving one feature set will degrade others; a complete slate of new big-budget titles will be limited and pricy, a long-tail library is thorough and "affordable" but lacks the biggest latest cheap, a low-price source may limit to second-tier content, a small distributor may specialize in obscure content for a premium.
NOBODY can provide EVERYTHING with COMPLETE satisfaction; there will always be room for a varying portfolio of content to offer, various delivery mechanisms which will appeal more to some than others, pricing which mirrors demand, etc. Just like grocery stores, each has its limits and annoyances and features and preferences.