Which it doesn't, any more than the power to tax for the purpose of general welfare (which is part of the same phrase as the "common defense") gives Congress independent non-taxing power to provide for the general welfare outside of the grants elsewhere in the Constitution. There's quite a lot of conditions, etc., you can apply to liability for taxes, etc., that allow some substantive regulation to be plausibly included under the taxing power, but if you read the purpose limitation of the taxing power to instead be a positive grant of power independent of the taxing power, as you suggest, that one clause alone would shift the federal government under the Constitution into one of universal plenary power with only negative restriction instead of one of specific positive powers, and its quite clear that that was never the intent of that clause (as well as that interpretation being completely inconsistent with the actual words of the clause.)