Why do real estate leads go cold in a single evening?
A buyer who messages about your listing at 9pm has usually already messaged two or three other agents about other listings in the same range, the same night. They didn't deliberate over whether to reach out to you specifically, they reached out to everyone who looked promising. Whoever answers first gets the showing. Whoever answers second gets a lead who already booked with someone else.
That's different from most buying decisions. Someone evaluating a SaaS product does it alone, over weeks. A buyer evaluates several listings in one evening, because good inventory doesn't wait and showings fill up fast. Your contact form isn't competing on the quality of its answer, it's competing on being first in a multi-way race, and a form that waits until morning has already lost.