I think it just comes from an obsession with having complex characters. Even down to minor, and maybe even one-off characters having to have some subtle nods and hints to a larger and possibly tragic story that may or may not even exist. (Where many crackpot fantheories come in.)

And sure, I myself love it when characters are well put together and have depth, but that doesn't mean you can't have a villain that's purely evil but still enjoyable to watch. It's all about how they're handled.

Because 100% evil/100% good characters are boring. Complexity and character depth are where it's at. Characters that are clearly human. Heroes with big flaws to show that they aren't perfect, and villains with redeemable qualities to show that they weren't born evil. A 100% good character can be a good role model, but when used for actual stories instead of just teaching morals, they are a Mary Sue, and impossibility in real life. A 100% evil character can be cool, scary (in a good way), and entertaining to watch, but can also be unrealistic as well, serving as more a personified force or living plot device to create conflict rather than an actual person with goals, personality traits, backstories, reasons for goals, etc.. It's why I'm a fan of grey/gray moralities, because they're more true-to-life: Nobody in reality is pure good or pure evil.