Why do writing guides say to just use 'said'?
Because "said" is close to invisible, and dialogue attribution is meant to identify the speaker without drawing attention to itself.
The mechanism. Readers scan past "said" almost without registering it, the way they scan past "the". That invisibility is a feature: it lets attention stay on the dialogue. A more elaborate verb — she expostulated, he ejaculated — stops the reader, and what stops them is the writing rather than the story.
The related problem: told-not-shown emotion. "'Get out,' she snarled angrily" tells the reader how to hear a line that should already convey it. If the dialogue and situation are doing their work, the adverb is redundant; if they are not, the adverb will not rescue them. This is the substance of Elmore Leonard's rule about never using an adverb to modify "said".
What is legitimately used alongside "said":
"Asked", for questions — equally invisible.
Verbs describing genuinely different vocal production — whispered, shouted, muttered — where the manner is information the reader cannot infer.
Action beats instead of attribution. She put the glass down. "I'm leaving." This identifies the speaker, conveys manner, and does characterising work simultaneously — usually the strongest option, and the one that reduces reliance on attribution verbs at all.
No attribution at all, in a two-person exchange where alternation is clear. Attributing every line is a more common fault than using the wrong verb.
What is not possible: she smiled, he laughed and she grimaced are not speech verbs — you cannot smile a sentence. They work as action beats, punctuated separately, and read as errors when used as attribution.
The counter-case, fairly stated. The rule is style advice, not grammar, and it reflects a particular preference for transparent prose. Plenty of admired writing uses varied attribution deliberately, and some genres and older works do so as a matter of convention. It is a default worth departing from knowingly, which is different from not knowing it.