It is a splitting axe, not a maul.
The head does have a flare to it which helps to pop the logs apart, which a cutting axe does not.
Good design matters. Why do you split with an 8# maul and not an 8# sledge hammer? Because the maul is shaped better to split wood. Keep optimizing it, you'd probably get to the Fiskars design.
There may be an element of physics. Force = Mass x Velocity. If folks are swinging the lighter axe faster then a heavier maul, it offsets the weight difference.
But for me, and many others, the biggest reason is with the lower weight you get less fatigue and you beat up less on your joints from hoisting the 8# maul repeatedly. I'm splitting wood with no more, and probably fewer, swings then I did with an 8# maul...but I can make more swings per hour because the axe is less fatiguing.
There is a balance point, Fiskars makes an even lighter "Pro Splitter" that's 2-1/4#. It works great till you use a Super Splitter which has a 4-1/4# head. That seems to be a reasonable compromise to most of us between lighter then a maul, but more effective then the Pro.