They're not necessary. When babies are tiny and light, you just carry them along without any trouble. When they're too heavy to hold, they're big enough to walk on their own. Furthermore, they are all possessed by the minions of the ancient demon Unfold, who is said to curse plastic devices, rendering them unfoldable.
So for anyone out there who has ever purchase a stroller, you're wrong and have failed at life.
(Jessica, if you're reading this please don't murder me in my sleep. I was just kidding! Sorta.)