I've already gone into it in a sibling comment, but basically because I can rest my arms and hands wherever I want, especially when reading a book for hours on end.
If you ever watch long-ish video content on your tablet or phone, do you keep holding the device? Or do you put it down someplace where you can see the screen? Same thing, really. Maybe it's not for you. But I've seen enough people in public precariously balance their phones on their knees while watching Youtube that I know I'm not totally alone.
It could be for me in theory, I suppose. My ebook reader does have a "stand" option built into the case. But the screen size is only 6", and so I suppose I never found a situation where I'd want to prop the device a foot or more away from me, because then I'd be unable to read the text. And if I could read the text, the font size would have to be so large that I probably couldn't fit more than a paragraph on the screen before having to turn the page!
If you ever watch long-ish video content on your tablet or phone, do you keep holding the device? Or do you put it down someplace where you can see the screen? Same thing, really. Maybe it's not for you. But I've seen enough people in public precariously balance their phones on their knees while watching Youtube that I know I'm not totally alone.