One is more than none, so I guess the PHB has a point of sorts there. If Dilbert wants the PHB to go he could have Alice go over to the PHB and demonstrate her new Halloween hair/face style. That would sure as God made little green apples get me out of somebody's office...
Because some people feel trapped and simply can't get out. They may not have a savings to fall back on, or the time to be job hunting and maintain their own personal life and current job.
And many employers actually do their best to keep that "I've got nowhere to go" mindset engraved into the employees heads. I've had a few jobs that threatened me constantly over nothing as an "incentive" to keep working harder. Anything from belittling my ability to work at all, right up to being fired for random thing, no Unemployment, and given a bad reference. They even claimed they'd have no problem finding new employees if they fired an entire shift (One place couldn't even keep the 3 people in our shop long enough to find the other 4 the job actually required... It often took weeks, or months, to find ONE person. They've a horrid rep in the area and people know it. Hard to find workers if no ones willing to work for you.)
It made me work harder alright.. to get a new job faster, and eventually into the training I'm in now for far better work. This is why I like night shift, opens the entire day up to interviews and job searching.
But many people won't feel/see it that way, and will be frightened into actually sticking it out and continuing to be abused by their employer.
Anyone who speaks against this, regardless of how good they are at the job (and how much it will suffer if they're lost. The above place mentioned actually fired the only guy who was trained on anything in one part of the plant, and expected others to cope with it without any new training...) will often be fired eventually for whatever reason the company can pull out.