Because those scanners are not up to the task. Malware is not like olden days viruses. There is no technical difference between an auto-updater and a malware downloader.
Not even the virustotal thingy is up to the ask. I have seen malware here, that was not flagged there. Some of the scanners used cannot even properly process an archive.
There are several fundamental flaws in the system. At the bottom line is, that itch is not a safe place. It is only relativly safe and that it is a legit site might actually make things worse, since people are not on their guard as much.
Playing games in a sandbox helps a bit. The itch app coincidentally provides such a feature ;-)