Time to answer rhetorical questions yo!
Purple is the color of regality, and most of the villains who wear it are either royals, nobility, or very wealthy. It’s also the strongest wavelength in the rainbow (the next wavelength up is UV which is harmful to humans), and most of the villains are after power of some sort. Finally, and this really only applies to a few of them, it symbolizes magic and the supernatural, so it makes sense for the witches and sorcerers to wear it.
Royalty? Magic? Power? Villain? Purple?
i feel enlightened