Agencies and their clients are overwhelmed by the need to communicate and deal with one another — to plan, obtain approvals, carry out work, calculate ROIs, and readjust after results have been analyzed. It's talk, talk, talk, negotiate, approve, reconsider, talk, talk, talk.
Working with an online media agency is less of the one-off, arms-length ad purchase contract and much more of an ongoing relationship. To get the most out of the relationship, clients need to make investments in the relationship, such as possibly recruiting digital/social experts. These are costs not all those new to online advertising will foresee.