itch.io

I want to revisit Poptropica. I have fond memories playing the game, creating art, and interacting with other artists in the Poptropica fandom on Deviantart and Tumblr. That said, I'm not going to blindly download a file to my laptop. Is Apple being paranoid? Why did it fail to verify whether "Poptropica Legacy" is safe for my laptop or not?

I received a pop up that read: "Apple could not verify "Poptropica Legacy" is free of malware that may harm your Mac or compromise your privacy". 

While I doubt that there's actual malware in this, the popup makes me uneasy. Why can't Apple check whether there is or isn't malware here?