The term has been around since 1988, but it's not truly understood by many.
You can't buy earned media.
It doesn't arrive on schedule.
Earned media isn't free media, because the amount of time and energy and risk you have to expend to get it is hardly free.
It's like all the other things we earn. It is worth more precisely because you cannot simply command it to comply.
[An aside: throughout the history of advertising, ad agencies have rarely, if ever, bought ads for themselves. Worth noting that firms that would seek to help you generate earned media are much better at taking their own advice.]