Why do we all sound the same when we sing?

Mainly because we all share the same language.

Although I would say there are some slight differences in the tones of voices when singing, which experts would probably pick up :)

I've often wondered that. I think it's to do with trying to reach a larger audience. Some British accents are specific to a certain area and the singer might not be understood elsewhere.

Maybe certain types of singing, but if you listen to the Beatles you can definitely tell they have British accents.

When in that tone and pitch when singing, accents are almost impossible to hear. That is just the way the ear perceives things.