Knuth didn’t arbitrarily make stylistic choices, he followed long established conventions in mathematical handwriting and printing where possible. IIRC he expla

Knuth didn’t arbitrarily make stylistic choices, he followed long established conventions in mathematical handwriting and printing where possible. IIRC he explained this at the beginning of The TeXbook. In this specific case though, it’s not hard to realize that parentheses, braces, radical symbols, etc. two ems taller than necessary both waste a lot of vertical space and look like crap.