Amazon uses their own system and as poplar as Kindle is, a lot of other stores just don't offer ebooks in the kindle format
Waterstones, their e-books is in Epub format
Sainsburys ebooks are in Epub format.
No doubt there are others, but I have not really looked into it.
sites that offer free books and just because they are free, don't mean they are rubbish, most of them are in epub.
but that is not the reason I did not get the Kindle, i just did not think it was the right reader for me and i did not want to be stuck with a non-standard format that only one company supports.
All of these free public domain books can also be obtained in mobi format just as easily as they can be in ePub format. Also you speak about the kindle format like it is some kind of obscure format, but it's not really like that. Other companies apart from Amazon do sell it, although not as many as ePub. However amazon are not an obscure company at all. They are the most popular (sales) eink device and ebook vendor, not just in the UK but also in other countries. Also if you buy from another store which is not kobo (if you have a kobo) or not B &N (nook) you need to use adobe Digital editions and your computer to get books onto your device. To me that's just as time consuming as converting books in calibre.
People talk about this great advantage of using the ePub format due to its openness but I just don't see it myself. The first ereader I owned was a Sony PRS-300 and I still own a nook simple touch now. I don't really see the advantages of having all those other stores to buy from when most of the time they are more expensive than amazon or at best the same price. Some of them like waterstones are considerably more expensive. Amazon are very good at price matching too. If on the rare occasion I find that a book is cheaper on another site, within a few hours amazon have usually price matched it. Amazon are still cheaper overall for me and before agency pricing came into play the difference in prices was been greater I find it much more convenient to simply visit the kindle store on my kindle and buy the books that I want. Than to trawl around a ton of ePub stores, find the book that I want and then have to turn on my laptop and connect the device up and run adobe digital editions.