EMA isn't easy to do. It's not a switch. Tasks must be divided up between the GPUs present, and those tasks must be completed in sync. This takes a lot of planning, so even if an engine has support baked it, it still leaves a lot on the programmer. If this were easy to do, we'd hear of more games using it.
Being a programmer, I'll second that assessment. Just because a feature exists does not mean it's easy to use. For the software houses that do plan to use this functionality, you'll see in-house libraries built around baseline interfaces because it fits with the skill sets and design methodologies they use to develop software. Each company that does software dev, gaming or otherwise, builds to the skill sets they have or plan to soon acquire. You may very well see certain companies consistently excel at EMA out of the gate while others will slowly catch up.