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I know of two advantages to the wedge method. First, as UnCivilEngineer notes, you can tighten the joint when it loosens, which it can do from the stress of use and material shrinkage. Second, it allows you to replace the handle when it breaks. More of an issue with (historically expensive) metal heads & (historically cheap) wooden handles.I suspect tradition also plays a role. Modern glues are much better than tool makers had when hammers were first designed.