And they do. The classification criteria also doesn't have to be the same between the readee ans the MC. That actually gives MC more deapth and provides more layers and food for thought for the reader.

The reader can know and sympathize with the villain's reason for being the way they are. The MC doesn't need to know everything the reader does, or if they do, doesn't have to sympathise.

The only issue I see for providing deapth to every villain is the "unneeded info" that can distract from the story. But that goes for anything. Each member of a pack of goods that have one-page worth of scteentime tops doesn't need a their backstory or what kinds of milkshakes they like spelled out for the reader. But that only matters for stuff put down on the page. All that back history and taste in milkshakes can still be there and inform the writer as they are writing, even if they don't put it down on page.