"'only' at 22 points given by nature" >> this isnt true. The sa and pa are non movable and dont have upper and lower shruti is for a practical reason. Tanpura, which is a drone instrument has typical tuning of sa and pa notes. Singers typically sing with tanpura, to get reference pitch. If singer sings variation (shrutis) around sa and pa, it causes acoustic beats (https://en.wikipedia.org/wiki/Beat_(acoustics)) where the volume appears to vary slowly. For example, tanpura sa drone string is tuned to 139 hz and singer sings 140 hz then audience hears beat of 1 hz. The beats are unpleasant so these shrutis are avoided. There is no 'natural' reason here.