Not to defend them or anything, because we've all been in your situation for something at one point, or will be rather, and a standalone installer would be nice. However they do explicitly state right before you purchase, on the products page the following:
The Mac OSX version of RPG Maker MV uses Valve's Steam platform as DRM. When you purchase from this site, you'll receive a key that will activate the Steam version of RPG Maker MV.
So anyone who purchased for Mac expecting a standalone installer or wanting one was told full well that there wasn't one and that it used steam. For that matter, they really don't even need to release one since they stated it uses steam. But again, not to defend them or anything, just trying to point something out
Sorry, if I came off rather rude.