Choice as well as training. Let's walk through the specialties:
We'll start with the "broad scope" (and low pay) ones you actually have a shot at matching as a DO
...
FP: Head to toe, right? Well, no sx, refer almost everything but the real basics like flu, HTN, gastritis, STDs
IM: "All systems?" "Advanced" dx? Still no sx, still have to refer/consult fellowship trained colleagues often
Peds: actually one of the broadest scopes, but only in a select population, refers out sx
ER: stabilize patient, either floor or door them, then move on to another... no time for advanced dx or tx
Psych: deal with crackpots, treat "pathology" that is almost purely subjective
Path: can do a lot of stuff and every system, but pt contact consists of reading pt chart (often a dead pt)
PM&R: basically non-sx ortho... a PT who can prescribe Rx
Now come some more advanced IM fellowships or similar specialties which narrow your scope a whole lot more. By becoming the ultimate authority on a system and its pathology, you are also giving up a huge chunk of your scope. And despite what you may think, these guys wouldn't "hang up a shingle and practice as a GP" unless it was an absolute last resort...
GI, Uro, ID, Endo, Rheumato, Neuro, Onc
The surgical specialties...
OB: nice mix of clinic and surg, but again, a limited patient population and organ system
Gen or Trauma Sx: fairly good variety and income... if you can take the absolutely hellish hours
System Sx (neuro, CT, vasc, plastics, etc): interesting, but probably most limited scope of all specialties
Ortho: A+ specialty with clinic/sx mix, but most restrict their own scope after spine/hand/etc fellowship
The NPC or "lifestyle" tracks (mised well pretend don't exist since you aren't at a top 50 MD school)...
Derm: good stuff + nice hours, but you are down to one organ system, a lot of narcissistic pts
Rad: all systems, but minimal patient contact
Anesth: great hours and job... but for years and years? CRNA popularity also limits your income
Optho: fantastic hours, but if you thought neuro had a narrow scope... what does optho have?
...and that brings us to podiatry. When you think about it, DPMs can do pretty much anything pathology up to the knee... derm, tendons, bones (trauma, reconstructive, elective), sports med, wounds, peds, orthotics/braces/prosthesis, path, etc. There is room for subspecialty if one seeks it out. It might be a small region of the body, but it's all systems within those parts. All things considered, that means the scope is actually pretty broad. If a DPM does not choose to limit their own scope by choice or training, then they are basically a F&A ortho who will additionally take care of lower extremity derm, nail, and wound care. Pod really is a nice mix that has aspects of ortho, derm, ID, rad, vasc, and endo all rolled into one, assuming you choose to acquire and utilize all of the training.
The bottom line is that, regardless of what your degree is, unless you are a small town doc forced to tackle everything or a guy who wants to get sued, nobody does it all. You are limited by what you are trained for, and that's why a lot of the specialties (path, anesth, peds, rad, surg, etc) have fellowships. Everyone has their role because they are the best for those pathologies and problems. Each student has his/her own interests, aspirations, and strengths. Sorry, but you are not going to be House from TV... diagnosing everything head to toe, doing any and every procedure, and then assisting in the OR surgery on your own patients. There are referrals and consults for a reason... the knowledge base is too vast for any physician to have "unlimited scope" and "treat the whole body."
Go ahead and dog on podiatry if you like, but if it were an MD specialty, it'd be pretty darn popular. You will see in a few years that you will probably end up fighting tooth and nail with other MD/DO grads over the small handful of specialties that actually let you begin your career before age 35 and might eventually afford you a decent income yet a life outside the hospital. GL
