ROI difficult for social media - rubbish!

Why is developing an ROI model so hard for so called experts in the social space?

What's so hard - you figure out where you are before you start (how many uniques / widgets you sell now) and then figure out how much you're going to need to get there.

You start and see where you get to - easy?

Well apparently not - most brands digital strategy is a mess.

My sense is people are making a fuss about ROI on social media because they don't understand how to plan, to strategise or to think.