Certainly doctors tend to diagnose what they know. Chiropractors might believe that all symptoms relate to misalignment of the spine. They might treat ear infections and asthma with adjustments. If you are a hammer, everything looks like a nail. Patients with Lyme like symptoms could have a wide variety of illness. They could have other infections like tuberculosis, HIV, chronic hepatitis and others. They could have metabolic disorders like thyroid disease or diabetes. They could have rare systemic diseases like amyloidosis. They can have a variety of cancers. They can have sleep apnea and primary psychiatric disorders. This list of alternative possibilities is by no means exhaustive; it is off the top of my head.
It becomes more complicated when one looks at another collection of disorders: rheumatoid arthritis, lupus, MS, chronic fatigue syndrome, fibromyalgia and others. These disorders are considered autotoimmune or the cause is not known. Lyme doctors may look at this spectrum of disorders as syndromes rather than diagnoses. Lyme doctors will often connect the dots. They will try to see if Lyme is present and whether is might be connected to the various syndromes. Other physicians may treat from a different paradigm. Autoimmune disorders may be treated with potent drugs which suppress the immune system and control the disease but never cure it.
Lyme doctors need to be good clinicians who consider a list of possibilities when they assess patients. There is awful lot of Lyme out there there and most of it goes undiagnosed or mis-diagnosed.