remember when Microsoft was the first to ship a browser shipped by default and dictated the standards by releasing it as they saw fit in that monopolistic browser? (for the kids of today, devs from that day loved ie just like you love chrome today. IE invented the developer panel and ajax request, both things you live by today)
well, Google (the irony) sued and won. Microsoft had to remove IE from Windows, and browser vendors agreed to not force standards themselves. that is the only reason we have prefixes. it send the message "we think it's cool, but wait for everyone to agree on the best way to implement this".
without prefixes, Google, having the browser monopoly (since Android is the new windows) can dictate the direction as it see fits. just like Microsoft when they sued them.