First off, there are assumptions that full stack developers are 100% amazing at all points in the stack. That's not the case. I consider myself more of an operator this day an age, but I still could take AWS or an empty linux box and build and deploy a java/go backend with a VueJS frontend for something. It won't be pretty, I can promise that. At the same time, that's why I have a great time with the other full stack developer who is much stronger on the frontend/backend side and kinda doesn't want to deal with the infrastructure and deployment.
And from that follows something that has taken a lot of stress from me. If the company is small, it's fine to have a full stack developer deliver something. There will be deficiencies in the stack somewhere, but who cares, there will be a minimally viable product.
Once the company scales however, people can start specializing into the areas they are good at. Why should I struggle too much with a JS frontend if I can make the entire application faster by thinking about the database? Why should the mentioned other dev struggle with one of the many ways to deploy the application if they could spend their time on features?