Every distributor is going to take a cut of your profit, and they should. They are providing you with (and constantly improving and maintaining) a service that puts your products in front of hundreds of millions of people. They also download easy and automatically.
You could sell it on your own, but your reach is zero. And you have to do your tech support for people who can't save a file and upload it to their devices manually. You have to maintain your site. You have to do lots of work to get people to even aware that your site exists, let alone actually visit it.
If you did traditional distribution, which is a long shot at best, you'd have a lot more hands taking significant cuts, as well as recoupment and etc...
It's a nice system where you can upload almost anything you want, having removed every traditional barrier to publishing, and let it sit there and make money for you, while you're almost completely hands off. And all they take is a healthy and deserved cut.
If they take 30%, then 70% of a lot of potential sales is so much better than 100% of nothing. And there is nothing to recoup. There is no storing 5000 unsold copies in your garage, etc...