I'm sure there is a task about this already but couldn't find any. VisualEditor uses a dialog format in which the progressive button is at the top right, and the bottom row of a dialog window is used for secondary actions; in the rest of MediaWiki (also the rest of the internet, most desktop software etc) it's on the bottom of the dialog. This can be fairly confusing even for experienced users, especially in Monobook where the visual clues are weaker (T128577).
They're in the top-left (top-right in RTL) because that's where they are in mobile interfaces where the surrounding cues are less available. This isn't a "VisualEditor" design; it's a fundamental design direction for MediaWiki which was made in 2013 and is being (very) slowly rolled out everywhere. Note that MediaViewer also implements this.
MediaViewer has hand-crafted dialogs with buttons mostly at bottom (example); in any case its dialogs are rather hacky and not an example to be followed.
My understanding is that there are two OOUI themes, the one used in VisualEditor with buttons on top, and the one used in all other places in MediaWiki with the buttons at the bottom. Is there any non-VE interface where the buttons-on-top design was introduced since 2013? Or are we talking about geological-scale slowly here? :)