First, when they sell someone's content, they get 30% cut as a distributor. Assuming the thing sells. Which it mostly does and it is not Apple's business.
Second, when they will sell their own content, assuming similar prices, their margin will be HIGHER than 30%. Because they absorb some costs and can eliminate excessive margin on produce. They still will get 30% even if discount their own production (as distributors).
Finally, about the content that sells. Thats why you go to A-list to create content that sells, like Swift's iTunes exclusive album. It is not made by Apple, but Apple could produce such albums in future, its not big deal and with hundreds of millions or so iTunes accounts, plus deals to Amazon, Netflix, HBO, it could easily recuperate initial outlays of 5 billions. They even could tie it to its hardware, like iPod-exclusive Beatles album or something like that.
5 billions is a huge money for ALL Hollywood studios even combined (most of them will struggle to produce one megahit, costing up to 200 million movie in a year), but 5 billions is a peanuts for Apple. it is just like 2.5% if their idle CASH sitting somewhere in Ireland and earning probably around 5% in interest only. Using only half of interest revenue from that cash holdings Apple could produce 25 MEGAHITS a year if it wanted. Or more. But probably they will prefer cheaper TV series, in which case they could produce up to 100 TV shows annually each costing 50 million dollars and having 10 episodes each and still be in black, cause Apple employs best writers, directors, other A list creators. And with good effort those megahits would have return of around 100% (like spending about 200 million on Harry Potter movie, spend another 100 million on ads, earn about 500 billion (even more for HP movies), be in black and still get profit of around 100-200 million dollars.
With profit on megahits, you can easily subsidize up to 20 family movies, kids movies which cost far less and don't bring much revenue, and 2-3 flops, when needed. In 3 years, Apple could produce more entertainment streams than Hollywood and Bollywood combined. That could fill 5-10 TV channels running almost only Apple's shows of highest quality.
And it all could be iTunes/AppleTV exclusive.
Do you really want an Apple dominated world?