I guess many do, but many don't. I don't wear one. As someone said upstream, your health is a privacy issue. I'm very upfront w/my diabetes, but many others have a need to hide their condition.
Aside from job issues, many people just don't feel a need to advertise their health status to others. How many people with HIV/AIDS are walking around w/a bracelet on??
Many years ago when I was raised, it was considered a universal signal to an id bracelet when caring for an accident victim. I did not know that attitudes had changed. I can't imagine why anyone wouldn't want that to be instantly identified, particularly a diabetic going into shock. Perhaps you have some other means...and I hope you don't say it's in your wallet.