Could it be because a 757 or 767 operating shorthaul routes is highly unlikely to be anywhere near it's max structural take-off weight? They both eat up runway at MTOW for sure.
Typically a 757 on shorthaul route with a full load of passengers and bags will have a TOW (or TOM if you must) of less than 100 tonnes for a max structural TOW of 113699kg.
A 737-800 is likely to be operating much nearer structural limits and consequently nearer to field performance limits is it not?
I understand also that the 737-800 has performance issues when landing on short wet runways.