Well actually, what I'm seeing is more that reusability is never a requirement due to time constraints but that an agency has a tendency to sell previous work as a concept to new clients (GOOD!), but then makes the mistake to think that the previous work can be reused (BAD!)
And how would you apply scrum if the team members work on 3 projects in a week on average, and projects only last 2 weeks? That way you can neither have sprints nor backlogs. What is a single sprint in scrum is a complete project in most agencies.
All that undermines the ability to estimate time, unless you start defining everything in terms like:
'A WordPress site, with [n] pages, module A, module B and module C'
But I can already see the next project where it is assumed that module A can easily integrate with SharePoint and module B can also double as an ecommerce module, all of which throws the baseline estimation off.