The simplest answer is that we have a capitalist economic system and a private health care system. So supply and demand takes over and doctor's are able to charge what they believe patients will pay. However, most patients don't pay directly for medical services. Insurance companies do, and government insurance through Medicare and Medicaid does.
In these cases, doctors are continually told what they may charge for a visit, albeit indirectly. Medicare will only pay a certain amount for visits and procedures, and doctors that agree to take Medicare patients must agree to those limits. Insurance companies are famous for saying what they will and will not pay for, and this sometimes limit what services doctors can offer a specific patient. Either that, or the costs are passed along directly to the consumer.
Just to add to the first response, depending on where in the nation a physician practices, his costs differ greatly. Undeniably, it is much cheaper for a physician to maintain a practice in a state such as South Carolina than it would be for a doctor in New York City or California. When doctors open up practices, they have to pay the rent on their facilities (or the mortgage in some cases), and they have to pay all of their staff.
Secondly, if a person wants to go to a physician who keeps his or her overhead very low (meaning that he might not have the newest equipment or the most modern-looking facilities), that is that patient's prerogative, and he will probably get a better rate. That being said--because we do live in a free market society--if a doctor can attract more patients by keeping up with modern interior design and by having the newest of everything, some patients will be willing to pay more in order to be cared for in such a setting. That doesn't logically make that much sense, but humans are often guilty of judging someone's skills by his/her appearance or by the facility he or she works in.
This is not a question about science, really -- it is more of a question about politics and rights and economics. In the United States, at least, there are a few main reasons why there is no maximum limit.
First, there is no way of knowing what sorts of procedures may be necessary in a given visit. After all, a person might need a lot of care and there is no real way to know how much the upper limit might be.
Second, we in the US believe in a free enterprise system. If people are willing to pay a lot of money for a doctor visit, why should we prevent them from doing so? We do not generally believe in limiting the amount of money that people can make or spend.
To me, those are the two main answers.