Time between closing and applying is not the same as time between applications, especially for people who don't MS and don't have a single large purchase that gets their minimum spend taken care of in a few days.

For people who take a 2 to 3 months to do the minimum spend (perhaps using this card mainly for things they can't pay for with an Amex, while doing minimum spend on an Amex simultaneously?), and then wait another month to get the bonus miles to post, then even just a few days between closing one card and applying for the next card may actually be more than 90 days between applications!