Reason why bike scratched up in this location under crankset
My guess is that this was caused by chain suck. This is a problem where the chain doesn't roll off the bottom of the chainring as readily as it should, and the chainring draws it up toward the chainstay. It can get wedged between the chainring and chainstay, and take off paint in exactly this way.

This can be caused by worn cogs, a worn chain, a too-long chain, or too little tension in the rear derailleur holding the chain taut.