Why must I work with a rehab medicine specialist for back pain?

You don't. You can work with anyone you want to. That includes doctors (internists, neurologists, orthopedists, physiatrists), physical therapists, massage therapists, yoga therapists, chiropractors etc. No one has a corner on back pain.
You have choices. There are many doctors who can treat back pain. You can see a chiropractor, pcp, neurologist, physiatrist, pain specialist, spine surgeon ... For the back pain.