I believe it is because they sag over time.

I’ve done it before I can’t find a picture of it on my phone

It may be just aesthetics. A bench seat using only vertical pleats looks better than horizontal pleats without being broken up with any solid vinyl. It may also be an impression left from the seat cover days of the past ,that would use press roll and pleat vinyl which the width is only 54" not enough to be used horizontally.

the first ford mustangs and I think falcons used horizontal pleats on their seats.