I don’t want to be a full-fullstack developer

It is easier for the business: you need to check with one person and process of development does not get too complicated with ‘who does what’ issues, it also cuts the costs making a little tradeoff with quality and development time. Backend development is a complex field, including understanding of network layer, the way servers work overall, deployment, AWS/Google/Azure services(they are vital to modern web applications), specifics of server application language and framework, protocols used, authentication, database connectivity and setup and a lot of other things. Don’t get me wrong, I think it is good to widen up the field of knowledge and employ understanding of your parts surrounding to do a better job on your section, but making developers be Jacks of all trades directly influences code quality, choice of solutions and the future of the project developed.

Source: artur-martsinkovskyi.github.io