He spends most of his time fighting mystical threats and would-be transdimensional conquerors. That leaves him very little time to pursue a profitable career.
How does he earn his bread and pay his bills?
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This was addressed in a couple of early issues. In short, Steven really struggles to concern himself with temporal matters. He either forgets that he needs money, assumes he still has it (from his earlier years as a doctor) or just plain buys stuff on credit. Initially he had Wong sell trinkets and gems from the archives.
And eventually he just uses magic to make a big pile of cash to shut Wong up.
Strange Tales #151
Even with unlimited wealth Steven still doesn't seem to cope well with the grubby business of paying rent, buying clothes, etc and eventually hires Sara Wolfe as his secretary/personal assistant. He pays her (and his bills) with magical gold bars, presumably refilling the safe whenever it runs low.
Apart from being able to do magic? Like make gold, make cash money, reach though portals to take money from forgotten vaults? Or go to other worlds and pick diamonds up off the ground? Travel in time using the Eye of Agamoto to get the winning lottery numbers, or pick the best stocks that quarter?
Apart from that,
Dr Strange is now part of a tremendously ancient and evidently well-funded order of magic users, the Ancient Ones. They have excellent realestate holdings in major cities around the world (as seen in the movies), as well as thousands of artefacts (in all those glass cases) which could presumably be sold to fund any shortfall.
Oh, and plus, he can do MAGIC.