Why?
Because MS Paint is pretty much useless, safe for total newbs who have never heard of Paint.NET, and have never progressed beyond the most childish doodling, such as "Rage Guy" comics.
Once Paint.NET is installed, MS Paint remains the default editing program. So when you right click on a JPG, then select Edit, the program that comes up is still MS Paint.
There is no way to remove MS Paint like there was on Windows XP, which is the instruction given by snakebitex. (Wake up! This is a Windows 7 forum.) The uninstallation of MS Paint is not an option in "Turn Windows features on or off".
The only way to uninstall this pesky program then is to go into your Windows folder, search for mspaint.exe, take ownership of it, and finally remove it. Then the next time you select Edit, you can pick your favorite picture editor.