Because they think that someone is being paid to pick it up, so why should they (a) do their job for them and (b) do it without being paid.
I guess it's a variation on the old upper-crust belief that the nobs are pretty much obliged to throw their clothes on the floor because "that's what the servants are for".
Are we growing accustomed to the heatwave?
Has anyone else noticed their energy levels change as they've got older?
