I think it's common for a lot of reasons.
Part of it is probably that villains can do things without needing to defend themselves or apologise. They get to be flawed and broken and weird without needing to compensate for it in any way- and are therefore more relatable to people, who also tend to be flawed and weird and sometimes broken. It's easier to see aspects of yourself in them - and maybe live vicariously through this character who can just go ahead and BE that flawed without having to worry about it.
In many stories (not saying this is the case with yours, though!), heroes get a lot of their edges sanded down, and their flaws aren't quite as honest and problematic as those of the villains, because creators want readers to root for their heroes, so we try to make them more appealing. However, sanding down the edges can mean that the result isn't as interesting; there's nothing to latch onto. The hero becomes blander than the villain, and that makes the villains more memorable.
Because unless it's done for comedy (I remember reading a comic focused on the Dark Lord of some terrible castle, and his problems with recruiting new henchmen, etc.), I often find that villains work better when you view them through the protagonist's eyes.
Centering the plot on the villain, and telling the story from their point of view, it's so easy to slip into one of two traps - either exaggerating their villainy to the point where they become so horrible that the readers don't like them, OR trying to sand down their edges a bit to make them more sympathetic, because now that they're closer to the "camera", you want to be sure people like them despite the flaws - and then we're right back to the issue we have with protagonists. No edges, no interest, you know.
