Then your imagination is limited. It's more a function of the entire medical experience. Sick people are grouchy. Busy places with waiting rooms make people grouchier. Few doctors provide immediate relief, which means their patients often leave just as grouchy. And upset customers are drastically more likely to leave reviews than satisfied customers. It's just the nature of the business.
People (including doctors with regards to themselves) are terrible at judging health outcomes that don't involve a transition from "I'm going to die" to "now I will live" - the biggest predictor of patient opinion of the quality of their medical care in a hospital, for example, was always how nice the room was.
Conversely, many practices will do everything they can to maximize physician utilization, including scheduling lots of back-to-back appointments. Inevitably, if a doctor is actually doing a good job, some visits will uncover more complex health issues than planned and require more than the 15 or 30 minute allotted to address. This means that appointments start to back up, people start having long wait times to see their doctor, and patient dissatisfaction ensues. Again, this isn't a problem with individual doctors or practices-- although it may seem that way to patients--it's systemic.
People who have bad experiences tend to look for an outlet, and there aren't many ways to "punish" your doctor. The good experiences are never thought about again...
That being said, both my current doctor and dentist were chosen partially on the basis of Yelp reviews. It's been a huge help, far nicer and far better service than I've ever had previously.
A couple of my family members have put their medical and dental offices through the Scheduling Institute[1]'s on-site consulting. The day starts with an institute trainer picking up the phone book, and calling several competing practices in the area to try and make an appointment.
Invariably, the receptionists are brisk, and often outright hostile. They get long hold times, and very poor customer service. And that of course doesn't end once the patient actually makes it to the practice.
There are reasons behind this (overworked providers and office staff, a limited marketplace, poor customer service training), but the result is even if the actual provider gives great care, the overall patient experience can be pretty shitty.
The other challenge I've experienced with my wife's medical practice (which puts an emphasis on customer service) is that patients don't really think about reviewing their doctor on yelp for a "good" experience. We do patient satisfaction surveys, and while the vast majority of patient's who respond to the surveys are very happy with their care, very few will leave a Yelp review about it.
We do just about everything we can without outright asking for Yelp reviews (check-in offers, prominently linking to Yelp from our site etc.), and still only 0.2% of her patients have left a review on Yelp, and only about 25% of those reviews have made it through Yelp's filtering process to actually post.
The other contributing factor is that older people use healthcare more than younger people, and older people aren't as likely to use Yelp. When I have a positive experience at a local business, I'll leave them a review on Yelp as a way of saying "thank you." If my wife could trade every jar of jam, or tray of cookies she got from a grateful patient for a positive Yelp review, she'd have hundreds of 5* reviews and I'd probably be a few lbs thinner.