The parentheses are large glyphs that distract the eye, and do not stand out (to the untrained eye) as delimiters.

Consider:

  (defun split-line (line)
    (remove-if #'(lambda (word) (string= word ""))
               (split-sequence:split-sequence #\Space line)))

Now imagine if we had some "lower weight" glyph besides the paren.

  .defun split-line .line,
    .remove-if #'.lambda .word, .string= word "",,
               .split-sequence:split-sequence #\Space line,,,

Obviously a contrived example replacing () with ., but you can see how "heavy" the parens and how they can dominate what the eye sees.

With experience, the parens vanish. The parens being large and common take control of the conversation more than they should.