This is an awesome question!
Imagine you’d need to first sketch a magazine on paper or as photoshop files as a designer, in order to have someone code a Postscript file, that can be distilled to a printable PDF - if there was no DTP application like InDesign, that allows a designer to take charge of the implementation just as the conception.
Let me share some of my insight into the industry from my perspective:
We’re experiencing a HUGE shift from corporate work to freelancer, small business and home-based business at the moment and have been for the last couple of years. Not only, but women in particular are leaving the compliance and constraint stricken 9-5 job to the advantage of a life closer to their family, of independence and creative freedom.
The client relationships of such a solo-preneur are distinguished by a very close one-on-one collaboration with customers who again are mostly small business owners and not large firms. This not only limits their budget but – what is MOST important: also requires immediate communication, quick response (within a day at most, and within one to two hours during active collaboration).
Over time, these designers have collectively accumulated a HUGE customer base by working efficiently and personably.
In short: Especially during reviews of the implemented design on the actual website, a designer needs to be able to respond quickly to customer feedback and amend layout, design and content while they have the customer on the line and providing instant feedback.
Constantly having a developer in the loop for every bit of adjustment not only adds to the bottom line money wise but also time wise – leading to a decreased customer satisfaction, which has a direct impact on the customer’s readiness for a referral and thus on further business opportunity.
Every now and then, solo-preneur web designers need the help of a developer, even on a platform like Squarespace, and they are glad to have a good work relationship with a developer they can trust and rely on.
While the workflow of first designing a visual and having it implemented by a developer then, is applicable in rigid design processes, – it is very inefficient in the context of increasingly growing numbers of profitable small businesses.