This has been in the tracker for 5½ years: https://github.com/mypaint/mypaint/issues/31
Since it is migrated from the old tracker it wouldn’t surprise me if the original is 6-7 years old or more, so it’s not like it hasn’t been considered and discussed (though not by me).
Although simplicity and an intuitive user interface are both important, those aren’t the main issues here. If these kind of tools were added, they could for instance be shown in the UI only if enabled in the settings (show extended toolset, or similar), thus preserving the simplicity of the UI for those who rarely or never need to use them.
The main issue is that adding selections and transforms, and doing it well, is a lot of work. It’s a feature that would span across a big part of the codebase; backend, frontend as well as libmypaint.
Personally I would like to have good selection/transform features be part of MyPaint, but it’s not something I am going to prioritize. If anyone wants to work on it, I’m all for it and will help answer any questions I can about how the existing code is structured, and what needs to be considered when adding these features (unfortunately I am still not fully familiar with all parts the code, but I’ll do my best).
In the meantime, we’ll have to make do with e.g. Krita’s (imo highly competent) selection and transform tools using the edit-in-external-app feature.