Why is XYZ feature missing?

Why doesn’t MyPaint have resizing, selection manipulating, color adjustments etc.?

While the project isn’t opposed to these features, there is simply too much to focus on right now. You’re welcome to implement such features yourself. Otherwise, if you consider such features to be absolutely essential to your workflow, consider using Krita or GIMP instead.

If you only use them for part of the workflow, you can change applications for these actions, see this question