There are a couple of reasons why wood burning might be banned.

Some towns are located in valleys that retain smoke, so if a lot of the residents burn firewood as fuel all the time, the whole town is continuously cloaked in smoke. That's bad for everyone's health. Forest fires are short term events that last a few days and then end.

Forest fires may have some benefits but they also destroy homes and kill people, and firepits might be banned where you live in case you accidentally start a larger wild fire, if you are in a high risk area. Controlled fires are only set in areas and at times of the season where they can be closely monitored and controlled and hopefully not end up creating a disaster.