You know, the ones who get bored once they realize they no longer have to work as hard to win your affections. Instead of rejoicing at the thought of the person they're dating being serious about and committed to the relationship, they actually miss being kept on their toes. They'd rather be kept in the dark as to what the person feels about them. Once their date is no longer a challenge, they go in search of a new one.
Playing hard to get might be acceptable when you've only just met someone and are trying to build interest, but once you've gone on a few dates with the person over the span of, say, a month or two, enough is enough. At that point, the two of you should be well past the games and focused on building a real relationship.
And don't count on the person ever changing. If you remain with them in hopes of their changing, you risk getting hurt or cheated on. Face it: some people are just too immature to be in a relationship.
Or, they don't like the person as much as they do the chase. When a person is really interested in someone else, they don't resort to such childish games. They have a genuine interest in making it work and are judicious enough to avoid any behavior that might turn the other person off.
Life is too short to get stuck with someone who doesn't share your life goals. If playing hard to get is something you both want, have a whale of a time. If you want something deeper, just wait for the right person.