Why are they called "nervous habits" even if I have nothing to be nervous about?

While there may be some exceptions, like your periods of boredom, mostly it is the sort of thing done by someone anxious or nervous. They can’t call it “The nervous habit for everybody but you” habit, so they just shorten it to suit the majority. :)

Picking your hair is not purely a “nervous” behavior, though it can be sometimes triggered by stress. In most cases such a behavior is just associated with other emotions such as boredom. In your case you learned that picking hair could relieve boredom and came up with your own self-soothing behavior.

You pull your hair out as in you have trichotillomania? Do you have any other compulsive habits like checking things multiple times, washing your hands a lot, anything like that?

Nervous habits are there to calm us down. Maybe you don’t feel nervous, because you are doing the habit? I do think boredom can also be a reason too. Next time you are doing one of the habits try to really ficus on what you are thinking about, or more importantly what you might be avoiding thinking about.

The term nervous habit” is old fashioned. Technically, habits like that are referred to as obsessive-compulsive.