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.)

A few things. Apple wants to control the visual narrative. They don't want icons to stand out because they want people to think they're using an apple device, not an Adobe product.

Companies see design as gold plating rather than something that gives a competitive advantage. This means they copy what's popular or outsource.

There's also probably fewer design firms working with these companies. This means fewer original ideas.

ubiquitous universal rounded rectangle

Squircle.