Corps can't go everywhere. If they did, there'd be no point in the trade game.
There's no restriction on who can refine fuel, and how much fuel they can refine, beyond time and the vagaries of the markets.
If one ship captain does it, it makes him money. When they all start doing it, that's when the market gets glutted and all the planetside vendors tell newcomers to just burn up the fuel for themselves because they can't accept more hydrogen - their storage tanks are all full to capacity.
Hydrogen isn't like hydrocarbons: it's not its scarcity that is the problem, but its abundance.
For instance, someone with a few sublight-only ships could convert a space station in the inner system, equip it with huge solar panels for power and a modest-sized fusion plant, have huge million-dt or even billion-dt fuel tanks, and massive fuel processors. He could then shuttle those ships to ice asteroid fields and gas giants, skim them for fuel and offer that fuel and energy to the passing trade practically for free, once he has recouped all his losses.
It's a matter of knowing how big the numbers are - the amount of energy being pumped out by a typical main sequence star, the amount of hydrogen there is the universe, and how popular the waste products are: iron, silicon, nitrogen, sodium ... oh, and oxygen.