Is there a legal reason why many advertisements say “Limited Time Offer”?
Sometimes, especially where the advertisement specifies the deadline. This would help defeating a claim of unfair and misleading practices that customers might pursue apropos of an open-ended offer.
An offer triggers the offeree's power of acceptance. Lapse of time --whether reasonable or specified by the offeror-- and revocation by the offeror are two permissible methods for termination of that power. See Restatement (Second) of Contracts at §§ 36, 41. By specifying a deadline, the offeror preempts both the question of fact as to what constitutes reasonable time under the circumstances, Id. at 41(2), and an argument that revocation of the offer was so arbitrary that it took customers by surprise.
Is there a law that says they have to include that?
No. The decision mostly depends on the offeror's marketing preferences and the litigation risks associated to making an open-ended offer.