I can’t speak for other cities, but in London many platforms are simply too narrow for a few feet of the platform to be cordoned off.

Additionally the other issue with overcrowding is emergency and disaster response. If the fire alarm goes off, you need to get all those people too the surface, without anyone being crushed or trampled (look at the Hillsborough disaster to see what happens when you get to wrong). I suspect that people being pushed onto the track is minor concern compared to what happens if people start to panic.