To louver your hood, you'd have to cut out a section of your inner reinforcement, then have the louvers stamped, and then weld the reinforcement back in, and hope that the hood held it's shape...
It's not that the metal on hoods is too thin to weld, it's just as thick as any other body part on the car, it's just that with hoods getting slammed and such, they're prone to having the body work crack, so most people won't touch them. Flat panels like hoods also present an easy opportunity for the sheet metal to warp, resulting in a lot of extra work to straighten it out. In body shops, they hardly ever fix hoods, just because of the liability of the repair coming back.