They haven't updated their Send To Kindle tool or Send By Email flow in years so those are still format locked for whatever reason to mostly only supporting mobi, despite that mobi hasn't been the current format published by the store in years and the current format is epub-based and the Kindles support epub just fine now (hence why a bunch of people saying you can just rename an epub file to something like png or txt and the Send tools work just fine with that because they are that dumb).