For someone who has been in this field for 20 years I remember starting out doing every aspect of webdevelopment myself. Then the bigger and more complex the projects became, the less that made sense.

The requirements of the distinct fields have gotten to a point where it's very impractical and hardly worthwhile for one person to master them all. You can't expect someone to translate a design into modern components that support all types of devices, resolutions and contexts to also come up with optimized SQL queries and knowing how to choose between Docker and Ansible for their distributed serverless functions that handle a variety of external endpoints.

Ofcourse not every project that uses this approach needs this approach, and there's still a market for the Do-It-All developer. My point is that the webdevelopment world of today isn't that of a decade ago.