A big part of that historically was a GUI toolkit might need to build up a whole object model, render model, various container types, event loop, timer queue, networking, IPC, file system abstractions, internationalization and tools to deal with that, debugging and visualization tools, etc, etc, etc. For cross platform toolkits a whole OS and build system layer to deal with that as well. At least the ones people ended up using widely (gtk, qt, win32, cocoa, etc).

That's to say that a useful GUI toolkit has typically been a whole lot more than simply "draw me some widgets and let me poll on input events"

I wonder if that's really still as true though in newer (not C/C++) languages like Rust for example, where you can reuse a lot more of what the ecosystem and language already provide.