1) You can't wait for a missed ADR, because there are still other waiting.
2) Sometimes, they can fit you in if there is slack time.
3) In other words, lots of people eat fast, so there are tables open.
4) Usually, the eatery will not let you wait as a walk-up.
5) They will simply turn you away.
6) Dining has just gotten so booked, walk-up is not possible very often.