I know snaps can be installed from the Ubuntu software. I'm just wondering if there's a way one can install a snap by double-clicking the file, and if there isn't if someone has tried implementing this and if not, why not.
Not currently. Installing snaps manually means you miss out on a few benefits of snaps. For example you won't get automatic updates if you install this way. There's also no guarantees that the snap you installed that way isn't malicious, as the store does some checks which you won't benefit from.
So that's a couple of reasons why I don't think any emphasis has been made to implement double-click-install of snaps.