THERE IS NO RULE!
Think of the possible permutations: you drop a chocolate on your clean carpet; you drop a lettuce leaf on your clean kitchen floor and perhaps rinse it under the tap; you always take your shoes off when you enter the house and you drop a sandwich on the clean carpet; you are out at the races and your kid drops his hotdog on the grass.
These situations are very different, but they all have one thing in common: the floor, carpet and grass are all places where people tread and possibly where animals tread too. Even in your clean house, there may be pets that shed hair and you have no idea which part of their anatomy is shedding.
If it's OK to do it at home, would you be happy to see food dropped on the floor in a restaurant picked up and put back on a plate?
I guess ultimately, it's a matter of personal opinion, at least in the domestic situation, but we know that microorganisms can transfer from one surface to another with amazing rapidity. The chances of getting an infecting dose of pathogens from eating food that has momentarily been on the floor is small, but is it worth the risk? What else is on the floor?
The difficulty, of course, is that you wouldn't give a baby food that has been dropped on the floor, but as soon as the baby starts to crawl, it will put anything in its mouth and thereby starts to build up immunity to bacteria in its environment. How do you decide?
For what it's worth, my professional opinion is that no food should be consumed if it has been on the floor.