In London that’s also mostly true due to the patchwork history of different companies building different lines… however when King’s Cross/St Pancras was redeveloped a few years ago the “official” interchange route between Piccadilly and Victoria lines became much, much longer - minutes of walking compared to seconds. This site doesn’t cover that station, but does link to TfL’s own diagrams via IanVisits, and the reason is clear: at one end the platforms of both lines are almost touching - and I believe that shortcut staircase is still there if you ignore the signs and know where to find it - but the tourist friendly route is much more circuitous, going up to the mainline station and back again. I assume it helps to relieve congestion in an extremely busy station, I remember more than one occasion when they just have to close entry to the platforms during rush hour due to overcrowding.

Why don't they block the shortcut to anyone but whitelisted people that applied beforehand?