Give it a little thought, the plumber uses copper to stub down from the plumbing fixtures but then installs non-metallic pipes between these stubs. Are those stubs required to be bonded together? No. then how does a water heater somehow mandate this bond? It doesn’t. As you pointed out it is just tradition from years gone by. Back before the introduction of non-metallic plumbing pipes there was a requirement in the NEC (1971 and early cycles) to make and keep metal piping systems electrically continuous but with the introduction of non-metallic piping systems this was removed from the NEC as the NEC does not mandate the installation of piping systems. This tradition is a holdover from those days simply because electricians are too lazy to study the book that mandates the installation of systems. They prefer to look at pictures published in some magazine and make their installations look like the picture. As the inspector on the job I must use the NEC for enforcement and am not allowed to use pictures I find including those found in the NEC Handbook.