Why is IM not seamless as email?

First, I would point out the main difference is synchronous betweem IM and emails. As for the technical part, I think the eqipments that IM and emails need are the same.

Instant messages are more interactive than emails. That means both sides should get online in the same time just like telephone. It is hard for me to be on the phone and do other things at the same point. So emails would be a better choice if I don't have time or it is not a right time to chat or talk. Also emails are not so intrusive and more efficiently. Take me for example, I can check my emails in the meeting. However, not possible to keep replying over instant messange.

But with the time, IM will definitely become more common just like home phone. Both of them have unique functions that are not replaceable. So the adoption is moving to the last stage.

However, the spam emails really result in inefficiency. So far IM doesn't bother me in that way.