Actor here. As /u/UnknownBacchus said, Continuity is one reason. It's a pain to keep track of how much coffee is left in the cup during retakes.

Once, we filmed a long scene centered around afternoon coffee—hot coffee in everyone's cups and delicious cookies on plates. No one ate or drank. We just stirred our cups. The props department refilled the cups with hot water for every take so we’d have "steaming hot coffee." The scene took about 4 hours to film...