Because the 250 hours aren't spend by one employee working fulltime for two months, but by different AD, IA, UX and PM people working together asynchronously. At the client, he'll be on his own, sharing an office with the bookkeeper and the support/sysadm guy who'll mock him for using a Mac.
Because at the agency the web-designer can work on 5-15 $15.000 projects a year, and spar with a bunch of similarly creative people continuously during the day. At the client, he'll work on everydays chores like putting press releases on the blog and updating the Facebook page, waiting for the few times a year when there's something that needs fancy layout.
Also, it's much easier to plan when you pay for a project, rather than having a guy on a monthly salary. And most agencies have nice retainer-deals so you can actually have a designer when you need him.
Because at the agency the web-designer can work on 5-15 $15.000 projects a year, and spar with a bunch of similarly creative people continuously during the day. At the client, he'll work on everydays chores like putting press releases on the blog and updating the Facebook page, waiting for the few times a year when there's something that needs fancy layout.
Also, it's much easier to plan when you pay for a project, rather than having a guy on a monthly salary. And most agencies have nice retainer-deals so you can actually have a designer when you need him.