There's also a bit of competitive pressure. Even if people get numb to ads, business, especially small businesses, can't afford to not show up if their competitors are still showing up in feeds.
The usual advertising psychology tricks still apply also which is why ads still work. Even if the ad itself doesn't result in a conversion, there's still the exposure effect of someone seeing your brand over and over again in their feed. The more times someone sees it, they'll subconsciously start preferring that brand or see it as more trustworthy. Among other tricks.
