An awful lot of street fights just kick off. There is not much thought in the mind of the aggressor period, let alone any idea that they will get their butt handed to them. The adrenalin surges and they lash out. It is explosive, usually uncontrolled and lacking in any focus.
However, you a right in the sense that if you KNEW you were going to lose you would have to be stupid or crazy to start a fight.
Bar fights in particular are generally not governed by this apparent common sense, as are any fights that involve any type of intoxicant or other drugs.
From personal experience as an ex-serviceman, drunks and drug users are dangerously unpredictable and lack normal pain reception (obviously). This makes them very difficult to deal with, and often the most effective strategy is simply to give them as much room as possible. They generally fall over by themselves eventually.
Fighting... it's generally not fun.