Thats a really odd spot for a crack that was caused by heavy pedal smashing, or rider weight.

Looks like a crack in the casting process and it could possibly be merely cosmetic.The aluminmu probably developed fatigue over the years and the crack eventually showed up ,For it to be on the Drive side on that braced/supported area is very rare ,usually the cracks develop on the square taper on the outermost corner. In that case its Junk.

If you have a Tig welder available you can bevel cut it to see the depth of the crack , lay down a bead,polish it back up and you'd be ok.
All this depends on the severity of the crack.

Crank arms usually do not develop cracks there, it could be a fluke.