Because everyone is always far too busy or can't make it, or they say yes and then drop out on the day?
This has happened to me once too often and for the last few months I have decided not to make any plans with anyone. If someone wants to see me then they can chase me and suggest meeting.
So far it's been fine; it probably helps that I'm not a hugely sociable person and so it doesn't bother me if I don't go out too often.
Does anyone else do this?