You know what?

they're learning communication skills, 'guess what' is a hook and since they're a kid they'll abuse it to get attention they want. at least until people (more likely other kids) start making fun of them for it.

People frequently use precursors in order to obtain your attention. It gets more sophisticated as people grow up but children find it effective early on. One of the reasons they are so enamored with it is parents tend to use that phrase with their children quite a bit to make sure they've got their attention and the kids just pick it up. So it's kind of like a baby talk left over

Adult versions are things like 'You'll never guess what just happened to me" Or "You're never going to believe this"

It creates both a sense of expectation and suspense. As adults, we have a lot more (and more subtle) ways of doing the same thing, but kids are still figuring out the nuances of these things.

They can't fathom that others know stuff that they don't.

Children tend to repeat what they hear, so I'd guess they must be hearing "guess what" a lot. I guess it's a good conversation starter especially if you're teaching them some fun fact or sharing a pleasant surprise.

Or maybe it's a call-and-response thing. When someone says "guess what" the listener probably replies with "what?" so it's almost like a game.

Depending on their age and development, sometimes they say things and don't really know what they're saying. My friend has a five year old, and I guess I must have been sharing a lot of stories with him because one day he busts out this gem: "When I was your age, triceratops has three horns."

It gets your attention and makes you engage. You say "what?" and now you're in the conversation vs. them just speaking when you may or may not be paying attention and give a a response.

It's just fun. Thinking back to my childhood, I think this phase probably comes in around about the time the word "poop" stops becoming the world's foremost source of hilarity.