.....you find it almost daily routine to be down alone a lot. In the mid-Atlantic the DM.s job is to tie off on the wrecks (usually over 100') and you use redundant gear. If they can't hook the wreck due to currents, you will be carrying the hook down with you. Then, of course, you have to untie the hook after the dive when everyone else is on board (and that means a free ascent with a sausage for the boat to spot you for a pickup).
As others have said, anything that goes wrong on a boat is the DM/mate's job to fix.....tangled anchor, a customer's lost equipment, a fouled prop, you name it. You bet the captain doesn't offer to be your buddy!
I think once you've spent a lot of time on your own down under to keep the dive boat going, it becomes more natural to be by yourself without the responsibility and confinement of a buddy. As captjohnr says above, recreation for working dive folks is frequently hunting. I couldn't imagine hunting with a buddy....although one or more other folks might also be down hunting at the same time.
Personally, I regularly take my own boat out solo and then dive solo. However, since I'm normally hunting I seldom need to exceed 50' or so. Some folks would say you shouldn't go in a swimming pool alone, but I would and do. To me, this type of diving is no different. Some folks wouldn't, but I do.