A lot of times for people like this, you're just plan B. They deliberately over-schedule or arrange their life knowing that xyz might happen, but let's just set up a date in case it doesn't.
Either treat your dates as they have 50% chance happening or ask your friend to give you an estimate and act accordingly.

I have friends like this. They’re incredibly annoying to make plans with. I stopped. Life has been so much better. I still see them but on my terms. Not theirs.