Why shouldn’t you vent the washroom exhaust fans into the attic or the soffit?
Venting the washroom exhaust fan into the attic allows warmer moisture filled air to be dumped into the attic. The attic most likely will not be able to deal with the volume of moisture by traditional ventilation and mold could form and make a mess of the attic and home. Similarly when exhausting out the soffit the moisture filled air get drawn back in the soffit (fresh air intake) and put back in the attic which creates the same circumstance as exhausting it directly into the attic. Best practices for bathroom exhaust fans include venting out the roof with a roof jack or really low on an outside wall a ways away from the soffit so the wet air has time to dissipate.