Did you ever throw a party and no one showed up?
Same happens sometimes to Ride Leaders who post rides.
Sure, plans may change. Maybe you’re not feeling up to it, maybe got stuck at work, or something else came up.
But if you see you are the only one who signed up “Yes, I Will Ride” but you can’t make it, a phone call/text to the Ride Leader would be a nice thing to do. Saves them the time and energy of getting to the start site and waiting around, only to end up riding by themselves or just bagging the ride altogether.