Marketing purposes. You know what's a huge gimmick that's easy to exploit? Parents, especially mothers. The moment a parent finds out their kids are different from the others, suddenly they're willing to buy a bunch of special stuff for them.

What do most households already have? Scissors. How do you make people buy more? Make them cheap enough to where parents buy them every new school year because it's cheap enough for you not to wanna look for your last ones.

Another way though is to create scissors that are "different" for kids, this way parents that otherwise would reuse scissors they bought last time will suddenly not be good enough for their child like the special left-handed ones are. Kids sometimes don't know they're left or right handed until later, but by then they probably already got scissors.

There is the off chance that parents will already know of course. Before they start writing in school, parents can tell very early on. How? Because, you know, drawing. At this point, because parents already know, the options are there simply a marketing thing, a feeling of personalization for their child, something the other competitor doesn't have sometimes. It requires the same machines to make them, so why not? If it makes your consumers feel, even if for no good reason, happier... why not ?

Tl;dr: consumers sometimes like personalization or buying items specifically designed for themselves, especially for their children. Because it's so easy to produce both variants of scissors since it uses the same parts and machines, it's only profit if the businesses exploits dumb gimmicks like this.