If I make a movie and sell you a DVD, the terms of the sale prohibit you from, say, showing this movie in a theater to a bunch of strangers and charging them money. Yes, you own the physical DVD, but you don't own the intellectual property of the movie itself. This holds true for physical copies, and it holds true for digital distribution as well. You are purchasing the right to personal use of the movie (i.e. viewing), and that's it.
If you want to e.g. show it in a theater (whether or not you're charging admission), you have to purchase that right separately. And that costs a lot more.
In the case of rentals, the company that is doing the rentals pays for the privilege of renting out the movie, and the cost they pay is predicated on concurrent access to the movie being limited.
When it comes to digital rentals, limiting concurrent access to the digital movie file doesn't make sense. Users who rent things digitally expect there to be no wait. But in turn, they're only given access for a limited time. For iTunes, that's 30 days since you paid for the rental, and then only 24 hours since you started actually watching it. You can't rent a movie on iTunes and watch it every day for a month, because you don't have that legal right. You must pay for that privilege, which is to say, you must buy the digital movie.
For Netflix, the limited access to the movie is gated by you having an active Netflix subscription. The moment you stop paying for Netflix, you no longer have access to the movie. This again makes perfect sense, since you're paying Netflix for the right to access all of their streaming movies for the duration of your active membership. You are not paying for completely unrestricted access.
Unfortunately, due to the nature of digital content, the only way to actually enforce these legal restrictions is by use of DRM. It sucks, but it's a fact of life. This is true for rentals, and it's generally true for purchases as well.