Most likely because of people who trace, or do what I like to call "obsessively referencing" which is just copying without tracing and saying it's original.
I would say it's more because they had put a lot of time and effort into their work. And it can be frustrating when you see some badly traced works, which are based on something you did. Though I always thought of it as a compliment when people wanted to use my works as reference of any kind. So... can't say I get them 100%, when they are against it. d:
I've heard of someone in New Zealand who tried to get a patent for the wheel. That thing was invented more than 10,000 years ago. I think it's safe to say the deadline has passed.
Those people may not be as paranoid as you think. You'd be surprised what some people would steal.