And on the other hand, any mail client can connect to any mail server, and any two mail servers can exchange email between each other, so the service is fully federated. The benefits here outweigh the downsides.
But also, why isn't there a commonly accepted better email standard yet? It's not because the RFC process is slower - it may be, but it's not that slow. It's because none of the big providers are interested in this form of interoperability. It's plainly obvious when you look at the sorry state of affairs in IM.
We're really, really lucky to have the interop that we have, solely because early days of the Internet were more collaborative, and enough of it got early adoption and stuck around, because it was too hard to uproot. But the more we go forward down the present path, the more Internet is splintered into proprietary bubbles.