Typical SFF motherboards have proprietary power supplies, proprietary front panel connectors, proprietary fan headers, etc.
So if the intention is to swap chassis, add power supply, and install a standard size graphics card, then the easiest physical thing to do is replace the motherboard.
Only real issue with that is Windows activation, so it can be cheaper to adapt the board rather than replace it. But if a used board is cheap enough, and Windows activation isn't a big deal, then it is certainly the option that requires the least amount of effort.
My argument is given current prices, you might actually be able to track down used Ryzen parts for less then you would think. There are going to be people dropping X370 boards in favor of X570 boards. Or people with low end B350 boards upgrading as well. And the 1st and 2nd gen Ryzen chips have been on a fire sale. Anyone jumping ship to 3rd gen is going to have 1000 and 2000 series parts waiting to be sold. DDR4 is at its cheapest point in the last few years.