Because most architects are still men?!!
Occasionally, 'they' see sense - I once went to a needlework exhibition at Olympia in London. The hall being used had 4 sets of toilets - 2 of each sex, but I'm sure you can imagine the women vastly outnumbered the men as such an event (I think most of the men were exhibitors!) - so they had made one of the sets of gents' loos into ladies' for the duration (there were no urinals - just cubicles, so other than the sign on the door and lack of sanitary bins and vending machine, the gents' was just like the ladies' anyway!). Still a big queue when the show opened though - all those coach-loads needing a wee!!!!