I think it's a matter of simplicity. When I want to revert a file, it means that I engage in my file changes that I don't want anymore and I want to revert. In photoshop it's even better cause the revert could be "reverted" in history.
I'll give you another user case more related to sketch. While working in product design, sometimes I am asked to generate a screen quickly with a different content, or without a specific feature. I open my sketch file, do my change, generate the design and leave without saving. All this in less than a minute. Autosave doesn't allow me to do so. Sometime, I need to generate a specific asset, and I need to flatten some layers, it's the same story, a quick action that I don't want to save.
In those cases, duplicate the file is very annoying, I don't need to do that. As a pro, working on design softwares for years, I took the habit to hit cmd+S from time to time, and I never lost my work because of that.