Why do some people get in the Carpool lane, then proceed to drive at a speed lower than the speed of the lanes of the regular freeway?

I’m guessing these people get into the Carpool lane because they “feel like it” and not much else. Maybe they enjoy the eliteness of that lane, with its wider shoulder and all that.

But shouldn’t the carpool lane be reserved for Carpoolers who want to use it in order to bypass the normal traffic that is too slow?

Don’t they completely destroy any real purpose the Carpool lane has, by getting into it, then driving at a speed equal to, or less than the traffic on the normal area of the freeway?

What can be done about them?