It seems like building a Twitter followers is just writing a lot of tweets

I wouldn't agree. Sure, it can help you, but what kind of followers do you want? Do you want the audience that will listen/read your messages, or do you want to have them engaged in conversations?

Depends on what you need those followers for. Different people have different needs. Depending on those Needs, strategy can vary a lot.

Volume helps. But you've got to be saying something that people connect with.