I think the main reason is they jam pack everybody like
sardines, so everybody will try their best to squeeze in.
Therefore, if they have sensor or side mirror and people are still
squeezing into the train, it will delay the train further, that's
why the doors have to close automatically.
Main reason is, they should increase the train frequency to prevent
commuters from squeezing which results in doors closing on
commuters.
The reason is SMRT, not the fault of the commuters, it's beyond the
commuters ability to increase the frequency of trains.
I would love to see people fighting when population reaches 6
million with the current transport system.