Our visual system has the ability to detect implied straight lines (and other simple geometric outlines) from very small clues.

Therefore "seeing" the vertical lines implied by the indentation is effortless - so it's immediately obvious which elements belong to each other.

Indentation is an incredibly valuable "brain" hack that manages to instantly communicate hierarchy, not something to be sneered at.

We have no such innate ability to match parenthesis - determining hierarchy in a jumble of open and close parenthesis requires precise counting or, typically these days, tool/editor/IDE support.