> I’ve seen and designed many decks for startups and some engineers pay zero attention to design.

"design" is such a vague term. In 99% of the cases the meat of the pitch is much more important than design, at least because of the thinking process that needs to lead there. Making beautiful decks without substance is going to be the risk if you hammer that kind of message, because there is always going to be more time you should spend on perfecting your pitch that will be taken by trivial design details. By going with very simple decks (little text, spare visuals, very few colors if any), you remove the risk of "bad taste" and this enables you to focus on everything else.