Why? Surely they are 'for' what their customers want to use them for?
That will vary from area to area, but in the end the managers/owners will do what works for them or close down. If someone issued people in my Wednesday group a ticket with a time on it we would just go elsewhere. We come from different towns, and many are reliant on buses getting in at different times, so whilst we aim for 11.00am, some arrive at 10.15 and others at 11.15. As I say, the owners are happy for us to be there - they push tables together in advance - as we are a reliable source of income. Passing trade is, by definition, unreliable, so it makes sense.
Maybe that wouldn't work in London, or in Windermere or other tourist places that rely on people resting their legs whilst taking in the attractions, but those managers will have their own ways of dealing with that. I don't think that one size fits all.