Three reasons.
First, the majority of mainstream carriers on the market do not have the baby sitting ergonomically at all, let alone when facing forward (and those that are more ergonomic still aren't perfect). When the baby is not in a seated position with their legs spread wide and the fabric of the carrier going knee to knee and their knees above their bum, it puts a lot of pressure on their little spines and hips.
There isn't much hard evidence for this though, but that leads to the next point.
Second, it's a comfort issue for both the baby and the wearer. Even the few carriers that are more ergonomic have the baby's body weight pulling forward on the carrier, changing your center of gravity, causing both the baby and the wearer discomfort in their backs, hips, and shoulders. Usually this discomfort can be fixed by doing what the person you replied to plans to do, which is to only face forward in short spurts.
Third, facing forward offers the child no way in which to shield themselves from overstimulation. When the child is facing you, they can bury their head if they are getting overstimulated or scared. That's just not possible when facing away from you and that can be very exhausting and sometimes even scary for a baby.