I think it's because it reads as "app", which is a more contemporary and encompassing conception than for users than just an icon or logo. Blame the very first iPhone for choosing slightly-Aqua-like roundrects for everything.

Yes, it looks weird to old eyes on the desktop, where the button-like shape is more familiar as a touch target, but we still recognize that they're apps.

It also allows the developer some control over the canvas that their arbitrarily-shaped logo is painted on, rather than just dropping it right on your user's wallpaper of their kids birthday party.

(As an aside, I'm on a Pixel that uses circles, but the Play Store (whose logo is a triangle) uses roundrects, so there's also a certain flexibility in app icons being a canvas within a platform-variable container shape, even if that's not a roundrect everywhere.)