Why are cinemas not FREE?

So many times I go to the cinema there are alwasy about 10 people watching a film and about 300 empty seats ... if the cinema was free then all the seats would probably be full all day, out of those 300 people at least half of them would purchase drinks, sweets, hot food, popcorn making the cinema more profit than it does when they charge £8 per ticket ...

It's a total rip-off £8 to watch a film ... so, why not make it free? Surely worth trying eh?