I've never used Krita, but I was skeptical that it can't do this, so I just now installed it and gave it a try. It does have this feature! I made a crop rectangle, then right-clicked on it. A little menu popped up, titled "Crop Tool Actions". One of the things in the menu is a checkbox labeled "Lock Ratio", which was unchecked by default. I clicked the checkbox, and then I could resize the crop rectangle with the mouse without changing its aspect ratio.
Took me under a minute to figure out, having never used the software before or read any documentation. I'd say that's one sign of a decent UI.