Because Gaia is a place where you can be who you want, whether it's real or not.
rolleyes As far as information on psi-vamps, there seem to be three basic kinds:
1.) Unconscious. These people really have no idea they're doing what they do, but other people may find them clingy and tiring to be around.
2.) Conscious Benevolent. These vamps know they have a tendency to swipe energy, but are aware and controlled enough to stifle that tendency, often choosing to skim a bit off everyone or find someone with an overabundance to balance.
3.) Conscious Malicious. They know it, and they get a kick out of sapping other people. They are often manipulative, needy, and just make your skin crawl after a while. At their worst, they may use their draining capabilities as a weapon (I've heard a couple stories.)
Generic Signs of Psi-Vampirism:
1.) You feel drained after any prolonged exposure to the person, especially one-on-one.
2.) The person seems clingy and anxious about being alone.
3.) The person manipulates you (or tries) to be around them more often, despite your desire not to.
Usually, a good psychic shield can help keep you stable, but nothing beats the mundane solution of avoiding the person. Do not confuse these people with empaths, who sometimes react to another's energy (say, they get more cheerful the longer they're at a party, but no one else looks tired).