Many trap guns have ribs that slant down in the front but many also have parallel ribs. The majority of skeet shooters want guns that shoot flat or very close to flat, i.e.55/45.
With skeet targets broken very close to 29 yards, a trap gun with a slanting rib that would shoot 4" high at 34 yards, would shoot less than 3" high at 20 yards, so the effect on the sight picture would be minimal.
Trap guns also have longer barrels than skeet or clays guns. 30" with 32" growing in popularity is the norm for over & under trap guns. Skeet gun barrel lengths were 26" but that has increased to 28".
The advantage for a longer barrel is increased shooting accuracy. The flip side of that coin is the wide lateral swings and greater swing speed necessary for skeet shooting and to a lesser degree, with sporting clays targets. Longer barrels are harder to swing fast.
As was mentioned, it is the height of the comb relative to the rib and the slope of the rib (if any) that affects how high guns shoot. A gun with a sloping rib cannot be made to shoot flat or 50/50 because of the slope of the rib.