I constantly wear earphones since I listen to audiobooks whenever I am not at work or home (read: in the car) but I do wear them when I walk for exercise.
That said, I assume full responsibility for my well being, and stay tuned into my environment visually, and never move to change direction or walk across a road without swiveling my head and looking.
Not everyone is as careful as I am, though.
Not being snarky, but how about the guy sneaking up behind you? I cringe when I see women blithely walking/trotting along with earphones on. Wife wanted to do that and got mad when I said "No". Changed her mind after watching a few episodes of "Forensic Files".
Using ear buds or similar ear listening devices while driving or riding a bicycle has been illegal in California for many years. The reason being because neither a driver or bike rider would be able to hear any warning sounds such as emergency vehicle sirens or the warning horns of other vehicles. It is simply not a smart thing to do if you value your well-being and that of others.