It's a controversial and complicated idea. The downside, and the reason why most doctors do not recommend full body scans, is that every human body is a bit weird and there will almost always be something "wrong" that will be visible in a full body scan. This can lead to unnecessary testing, anxiety, and even unnecessary procedures. Many of these oddities flagged by the scan would never have caused any actual issues had the patient never been aware.
While there are many individual stories of full-body scans detecting early-stage cancer before it became symptomatic, there seems to be a general sense among doctors that implementing full-body scanning on a population level would lead to overall more harm than good. The thinking is that it is better to do regular targeted screenings for diseases that you're in a risk group for (e.g. colonoscopies, mammograms, cancer marker blood tests, etc.) rather than full-body scans.
I'm not a doctor, and I personally do find the idea of full-body scans very appealing, but I also know that if the scan detects a possible cancer, I wouldn't be able to just ignore it if the doctor tells me it's likely ok. Any time I felt any pain or any sort of symptom in that general area, I know I would worry about it. Maybe that's worth it for the potential life-saving results, but it definitely is a cost of this type of scan that needs to be acknowledged.