Designer here: there's a trade-off between visual harmony (all icons look the same) and ease of differentiation.

A standardized container adds regularity to irregular shapes.

Recently, Apple has been heavily opting for visual harmony, so their icons look consistent when seen as a set. Google too. It's an industry trend that is fairly annoying.

Similar "let's remove the differentiation" decision made for menu icons in macOS: https://tonsky.me/blog/tahoe-icons/