Insurance and lawsuits.
First, few pet owners carry pet insurance. Therefore, every recommendation for their pet's care must be consciously accepted along with its cost. In human medicine, the patient has little ability to know the cost of any service in advance… just accept it and let insurance pay for most of it.
Second, fear of lawsuits cause human doctors to run many more tests, and have you undergo more expensive procedures than you may really need.
Ironically, these two reasons collide. Insurance won't let a doctor routinely order a comprehensive panel that includes CBC, organ chemistries, electrolyte levels, thyroid test, glucose, a urinalysis, a pancreatitis test, a test for heart damage, fecal parasite screen, and heartworm and tick-borne disease screen. This one panel for animals costs pet owners $350–400 and catches many developing health problems when they can still be addressed more easily and cheaply. Your MD cannot order such a comprehensive panel, instead choosing which specific tests are indicated based on symptoms, even knowing that this picking and choosing costs more than just running a standardized panel.