[paranoidrobot]

>just there to make the food feel freshly made

There's a significant quality difference, even with mass-produced burger patties, cheese and sauce.

Bread and meat don't respond well to the same treatments, and as anyone who's had a service-station pre-prepared sandwich knows - bread absorbs moisture and goes soggy/damp if prepared more than an hour or two before use. With fillings that need to be re-heated it gets worse. There are various ways that they try to fix this, generally by putting more fats/oils on.

IMO I would rather have them prepared separately and combined at purchase time, even if it is the same ingredients that would be on a pre-prepared burger.