What happens if the crossbow toting wizard runs out of ammo? How do you rate a crossbow sans ammo against that ray of frost? 50 feet is a darn sight better than the wizard could throw that crossbow.
A siege doesn't have to be a grand affair with an army, catapults, and a castle either. It could be a few guys (like a party) trapped in a dead end mine with a bit of cover at the entrance and a superior force outside. It could be in a city, with one side being trapped in a building with the exits covered by opposition that may be suicidal to just run out and engage.
The concept of a siege is simply being between a rock and a hard place and it is a common one in many genres.
When a particular magical effect is completely AT WILL with 0 resource considerations it affects the world in ways beyond the standard door/monster/treasure D&D assumptions.