Full stack implies hosting, networking, security, database management, server-side application layer, client-side application layer, third-party integration, and testing infrastructure.

Each layer has it's own set of technical choices, which includes language, framework, design patterns, vendor tools, and best practices.

As your site grows you have to deal with scaling your codebase, managing large data sets, refactoring, spam management, user support, in- and out-bound email, asynchronous services architecture, project management, and hiring technical talent, to name a few.

I have built a website with a very small team and have to work on every layer and issue listed above. It's vastly more difficult than building a desktop app for example.