Why do people say, “Let’s ____ together” but they don’t want to?

For example, you might be chatting with someone about a show and they’ll say, “Yeah we should hang out and watch it together sometime!” But then when I follow up on that offer they suddenly become the busiest person in the world?? Like if you didn’t want to hangout you didn’t have to say anything?

I assume the reason is because they’re trying to be nice, but getting someone’s hopes up for something you know won’t happen is the opposite of nice. And the way this is so common makes it hard to tell when a genuine offer is being made by someone.

Anyone else experience this phenomenon? Anyone do this themselves? Thoughts?