• Not everyone can do it
• Not everyone does it professionally
• Not everyone wants a reading
• Supply & Demand
• It takes time to prepare for your reading.
• It takes time to clean up after the reading.
• It costs money to replace tools such as Tarot cards and buy materials for your reading
• It costs money to read through a phone service.
• The reader might need to share profits to have a listing on a Web site.
• It can be exhausting.
• It's at the level of many other professional people-focused occupations.

Also if they were doing something else for money, they may not have the time, energy, or ability to read for others on a professional basis anyway. And it doesn't make sense to tell someone to work at something they may be less capable of doing just because someone says they shouldn’t charge for their so called “gift”. That means a lot of people shouldn’t be charge for the fine services they offer.  Or maybe those that make this statement are frustrated because they are unable to afford a psychic and don't want to work towards being able to either! 

It's my opinion that everyone ought to be paid (money, barter, or otherwise should they desire) for their talents and the law of supply and demand does increase the charge of a psychic's fee.

My Personal Reasons

My rates are based on my expertise (Certified Professional Tarot Reader who is accurate), breadth of ability (astrology, numerology, palmistry, tarot, pendulum, face reading, psychic, medium, etc), education (business, career & work counseling, and art), outside work experience (people manager, business analyst, customer service, training manager), inside work experience (over a thousand readings -- I stopped counting!! hehe), demand (there's a lot of people who want readings from me), availablity (I'm often scheduled 1-2 weeks in advance), office space if needed (it costs money and travel time), and many more reasons.