Because the OHP is a full body exercise, not an isolation exercise. The core should be doing a large portion of the work. If you're struggling with technique, the last thing you want to do cut corners by using a belt.
If you want to build just your shoulder strength / size, then a seated press is more efficient.
People seem to use belts as fashion accessories, they would be much stronger all around if core strength and flexibility was improved, then belts would be unnecessary.
Well I can press 25% more with a belt, compared to the point where the weight makes me mentally think twice. So the extra stress is likely to be making me stronger overall. Including my core and legs which is going to be more effective than a seated press.
Because the OHP is a full body exercise, not an isolation exercise. The core should be doing a large portion of the work. If you're struggling with technique, the last thing you want to do cut corners by using a belt.
If you want to build just your shoulder strength / size, then a seated press is more efficient.
People seem to use belts as fashion accessories, they would be much stronger all around if core strength and flexibility was improved, then belts would be unnecessary.