A lot of people keep saying, that its to avoid collusion by someone telling their dead cards in the chat. But this makes no sense, since information about dead cards is equally valuable, weather someone is all in or not. Lets say for instance, that player A has open raised, and player B selects to 3-bet. Player A have 1.500 chips, player B have 10.000 chips. Player B wants to move player A all-in, so he 3-bets to 1.500 chips. He dont move all in, because he still have players left to act, who also have around 10.000 chips.
It folds around to player A, and the chat is not disabled, because nobody are all in. But player A still need to make a decision for his entire stack, so its just the same as if, player B had 2.000 chips and had moved all in. If player A call however, THEN the chat is disabled, even though nobody have another decision to make in the hand. So clearly this has absolutely nothing to do with preventing collusion. Most likely the real reason is, that poker sites think, people should sit and watch board runouts with excitement and not be distracted by the chat. Which is just kind of silly really

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The situation you described though would happen far less often, at least on the later stages of a tournament or SNG.