HA! Because they don't. Not without a raised runner and a bunch of work. A decent prepped W-2 will beat the **** out of ANY 23* standard Chevy head.
Again, people are caught up with flow numbers. I remember when a certain engine builder guru was advertising his head as the first head to go over 500 CFM. If you read the fine print of the ad, it didn't do what he was saying, but no one cared because they wanted the advertising $$$$$$$.
Fortunately, like an STD in at a trailer park, other head guys caught onto what was happening and made the fix the add.
CFM does NOT equal horsepower. I know that's thought to swallow, but a well prepped port may flow less on the bench but be faster on the track.