Reason being TIG is set up to maintain a constant current while MIG welders maintain a constant voltage. Different beasts. Arc welders are constant current so you could convert one of them into TIG.
Though I've no idea what would actually happen if you started from a MIG. It'd weld badly I guess.
I will probably weld but you will need to hold the torch real steady as arc length will vary current significantly.
And scratch start on a MIG supply will be a massive current and contaminate the tungsten.
I did a bit or arc welding with my "riding welder" set up for MIG. Worked fine, but needed careful control of arc length for good results (have since added arc mode to it.)