If you want your ebook on Amazon, then Amazon will take 70% (The 30% might be available to you if you plan on retailing at a low price and in mainstream markets, but an indie author serving niche topics will often want to set a higher price; Also you can't do public domain content on the 30% option).
70% commission strikes me as being extremely high. Compare that to bandcamp, which takes 15% for being an extremely well-known platform for indie music.
Competitors who try to eat at least a little of Amazon's lunch (like Rakuten's Kobo, or national book chains like Barnes & Noble's Nook in the U.S., Thalia's Tolino in Germany etc) are following Amazon on pricing. Only outsiders like Google & Apple offer better deals, but those platforms aren't all that interesting for e-books.
Of course you can try to sell e-books by offering downloads from your own store, but you'll have to work a lot harder and spend more on marketing, plus I'm not sure that, outside of specialist techie audiences, people would be willing to endure the friction around downloading an EPUB, connecting an ebook reader via USB, transferring the file etc.