They both lack advanced features that many drawing applications (Krita/Medibang/CSP), such as a complex brush system, advanced color palettes, Stabilization, Mirroring etc. Photoshop/GImp: The reason those two are referred as editing software is because they were made for editing primarly with filters and tools, and the painting/drawing functions are more of an afterthought. Lot of beginner still use it because its an easy way to get into digital Art because of the simplicity It can be enough for some people, homewer it is very limited and sooner or later you will need to use other software for stuff like color management, etc. It is very popular because it is very easy to use and can run on anything, even decade old pcs. Paint tool SAI (not free): Of all the painting software this one is the simplest. I had performance issues with it but maybe it not the case anymore It is primarly for drawing, there are no advanced editing tools like in GIMP/Ps. Mypaint: A rather simple software, with an infinite canvas. Here’s a shot summary of the difference between each software: (quotation marks because Krita has some room for improvement on all of those, despite being very powerful in other areas) for that but “just” solid tools for clean outlines, filling and smooth shading. Reason may be that it doesn’t take super powerful brush engines etc.
It seems to be very minimalist, and I mostly see it used for anime/manga style artwork. I don’t know too much about SAI either, the 1.x release is super old, and 2.x seems to be an eternal beta (probably not acrtively developed anymore?).