Why are PhDs the most common doctoral degree? I have heard many people say they have a PhD in a certain area.  For nearly all of my life,  I thought a PhD was simply a doctorate,  and I didn't know that it had anything to do with philosophy. 

There are many doctoral degrees, why are PhD's the most popular doctoral degrees by a long shot?
This answer suffered a Major Revision!



A Ph.D. is an academic doctoral degree and have nothing to do with philosophy nowadays. 

From Wikipedia: 


  In the context of the Doctor of Philosophy and other similarly titled degrees, the term "philosophy" does not refer to the field or academic discipline of philosophy, but is used in a broader sense in accordance with its original Greek meaning, which is "love of wisdom".


People often refers to an academic doctorate as a Ph.D., even when the official title in their university/country is different of Doctor of Philosophy. The reason beyond its popularity is because a Ph.D. is a title traditionally conceded in countries with anglo-saxonic language and since the common language in the academia is English we often "translate" our titles to the equivalent in English. 

I have recently learned with @Pete L. Clark that there are different types of doctorates in the US that give the same legal statute as an academic doctorate, but with different structure and requisites from a doctorate in Science. The example that he used was the Doctor of Arts (D.A.), a title that gives equivalent rights as a Ph.D. in which concerns teaching and research in universities. 

My previous misunderstanding was due to the structure of high level education that I am used to in Portugal and Brazil. In both countries Therapists, Physicians, Lawyers and others do not have a doctoral degree. In Portugal a physician have Master's degree in Health Science or a 6 years pre-bologna licenciatura degree (undergraduate degree). If a physician/lawyer/therapists wants to have a doctoral degree here they need to engage in a minimum 3 years doctoral course, publish articles and defend an original thesis. 

For a historical insight and more complete overview, please read this article and this article.

I'm sorry for the previous wrong answer. I think the question is too wide and depends highly on which country we are talking about. I assumed things about the doctoral degrees on the US that are not true.

Edit: Of course that there are many different ways to refer to an academic doctorate depending on the country the title is emitted. The point is: they are all equivalent. A German Doktorgrad is equivalent to a Ph.D. in the UK and to a Doutoramento/Doutorado in Portugal/Brazil, etc. In the other way researchers would not be able to work across the borders.