Why are they Tanks: Fives, Old Ben, Teebo,...?

They are tanks because GoH doesn't have a Bruiser category. There are bruisers in the support and attacker categories as well.

Bruisers are designed to off-tank and punish opponents for not dealing with them. They tank not by taunting or other forms of hard CC but rather by pulling aggro because the opposing players has to make a choice.

Bruiser have been made mostly irrelevant by offense scaling far past defense and speed being king. Anakin is one of the few exceptions due to his TM and damage gain when he's ignored.