Why won't hostesses clear lingering customers at tables in order to seat new ones?

It irritates me when I have to wait over 30 minutes on a table and I can clearly see customers who’ve already eaten and paid but are just sitting at their table chatting. I’ve been told by hostesses that they’re waiting for the customers to leave and we would have the table next. Doesn’t it make more business sense for the hostess to politely ask that the lingering customers move to the bar area etc. so new customers can be seated? It seems to me they’d rather take a chance on pissing off a customer who has already paid and is rudely lingering in the restaurant than one who wants to sit down and give them their money! Someone please clear this up for me!