Because Facebook leads are interrupted browsers, not searchers. They tapped a form between vacation photos. Instant lead forms make it worse in one specific way: prefilled fields mean people 'submit' with two taps and half of them barely remember doing it.
Fixes that raise quality at the cost of volume (a trade you usually want): add one qualifying question to the form, switch 'more volume' to 'higher intent' in the form settings, send clicks to a landing page instead of the native form, and say the price range in the ad. Expensive-sounding copy filters bargain tourists free.
And answer fast: a Facebook lead cools in minutes, not days. Half of 'bad leads' are good leads contacted late.