Because at the funeral, when Dick had said there was a piece of Wally inside of all of them, ... In Artemis' case, he was definitely on the right track.
My headcanon is that Artemis got pregnant the night before Wally ceased.
I've seen a lot of theory's on what their child would look like and I know it's completely un-creative but I just sort of pictured Wally Jr. being an exact copy of Wally Sr.. I also believe that even though he didn't get to meet his dad, he's heard enough from his mom and ole' uncle Dick to know that his dad was totally awesome and wants to be just like him.
... Sorry about the feels.
... And the crappy picture in general, I'm sorry.