I guess it feels plausible because it sounds similar to how advertising works: discount supermarkets and dollar stores intentionally try to look cheap and tacky

I guess it feels plausible because it sounds similar to how advertising works: discount supermarkets and dollar stores intentionally try to look cheap and tacky because it makes customers expect their prices to be relatively lower than the competition (whether that is universally true or not). They specifically want to attract people looking for cheap, so they try to look cheap - in their branding, advertising, print materials, etc. Of course scammers aren't literally looking for people trying to be scammed, so the comparison breaks down there. Plus of course there are other strategies than just spray-and-pray (where lower upfront costs by not investing in good copy or plausible design may actually be a good strategy because you can't handle too many marks at once) and with many of those better quality investment definitely increases the chance of success (e.g. spear phishing or whaling).