The first thing you do in trauma focused therapy is establish safety, then build up tolerance and recovery techniques. You're going to have to give some more information as to what you mean.
Therapy and exposure are both helpful, but not all exposure is theraputic. If someone is actively treating their trauma, containment and building up a sense of safety is one of the first steps. We don't treat soldiers with shell shock by sending them back to combat zones, nor by repeated showings of violent war movies that aren't intended for therapeutic use, and not in a theraputic context. That isn't to say that any exposure to a concept is inherently negative, its surely one of the better treatment methodologies that has evidence behind it. But the context in which things comes up does matter. Its the difference between someone making progress towards healing and going into a negative coping mechanism spiral.
Lots of people use trigger warnings to avoid topics entirely, and yell at people who don't use them with anything even remotely related to the topic. They demand everyone elsee change rather than treating their own disease.
Most public settings—such as a classroom—where a trigger warning might be relevant do not meet these criteria.
The problem nowadays is that most therapists simply don't want their patients to get better.
Once you help someone recover, the revenue ends, not just for you, but also for purveyors of fine psychiatric medications.
It's never stated that succinctly, of course, but that is absolutely the result. It takes real work to find mental health professionals that actually want to heal their patients.
Its like saying firefighters don't want to put out fires so they'll keep being paid.
Burning houses don't yield endless revenue streams for fire departments.