That’s one reason. The other one is that when you have a small amount of total milk, you have to make sure there’s enough for everyone. By pouring milk first you can more easily measure how much milk you pour per cup.

Good tea cups would never crack from boiling water, unless they were frozen first. Tea first is probably due to the fact that everybody wants tea, but not everybody wants milk, and some people want to pour their own amount of milk. You offer tea, they accept, you pour tea, you offer milk.