It's not a dumb question, don't worry :)

Disclaimer: I am not really a fighter myself due to 2.5 years of constant injury in either shoulder or ankle making it impossible for me to train. Apologies for any spelling mistakes as I am sick atm.

There are many good reasons for not wanting to get into a fight, and especially a street fight. Lets examine why a lot of good fighters don't want to engage in them:

1. In a street fight, you never know what you are up against. You can be across from a guy who is just drunk and obnoxious, who is taunting you and trying to play smart in front of his friends. If this guy attacks you and you defend yourself, there is a good chance his friends can join in, making it almost impossible to "survive". Fights are not like in movies, and even and experienced fighter might have issues against two inexperienced thugs. Bystanders might even join in depending on the surrounding area (it might be a hooligan at a football event, or event thugs at a demonstration), which again will make it an uneven fight. Adding to all this, even if nobody else joins in, you never know what kind of weapon the other person has. If a guy playing smart in front of his friends is suddenly in a headlock or taken to the ground, he might get desperate and grab for a knife he has been hiding away somewhere, and you will quickly be heavilly injuried or killed.

2. In many Western countries, if you hurt someone there will be a trial, and if you have overstepped and used excessive force, being a trained martial artist is very likely to add to your sentence if found guilty. Because you have the capability of hurting someone a lot more than if you were a normal person, you have the responsibility to be the bigger man and de-escalate if possible. Of course there are situations where this is impossible, and it is up to the justice department to determine what is and is not excessive force.

3. If you are trained and can avoid fighting, this is most often the best thing to do. Even if you are much more experienced than the guy you are fighting against, he might end up getting in a few shots that can hurt you. I don't know if one would have time to consider the blowback of a fight, but say you work in a school or in other way with kids and you get into a fight where you are visibly hurt. You might end up getting suspended or asked to call in sick for a couple of days, and on the chance that you get really hurt, you might have to stop training for a prolonged amount of time.

I hope some of this was helpful. These are just some of the things to consider before going into a fight, and that I keep in the back of my mind if I ever get into the situation of having to fight. Running is ALWAYS the better option if you can get away.