Sorry if this sounds like a dumb question, but I'm kinda new on this site and it's been bugging me for a bit. Is there a specific reason why all item icons have a white background behind them instead of being transparent?

i don't think there's any specific reason other than that's just how it's always been since the start of the site. at the very least, it makes the item clearly visible and delineated on any background, because if it had a transparent background it might be hard to see on patterned or dark backgrounds.

Primarily because then they can stand out better on any background put onsite. That's my understanding.