What's up with the people who constantly suggest hanging out but never actually make plans?

So many people like this that I've met. Every time you see them they suggest hanging out, like actual plans, but it never happens. I've even messaged this specific girl, that constantly does this, if she has time this day, that time for this specific thing, and she just never answered. And she's the one suggesting in the first place. At this point I just give up on these people, but I don't understand it.