Correct, but I-bonds cannot be cashed out in the first year, and households who can't afford to save more than $10k/year probably aren't interested in having their investment tied out for 1 year.
Quite frankly, a S&P 500 index fund would serve them better than an I-bond, even during severe market drawdowns.
$10K is way more than what most people can save in a year, so it's effectively unlimited for more households.