Systems like Flatpak have a framework system where it is possible to have a shared runtime of dependencies that multiple applications can use. A GTK1 runtime could exist (or maybe does?), for example.
Or perhaps more fitting to modern Linux desktop, Gtk1 and Gtk2 will not support Wayland nor any new functionality introduced for scaling (did you know that, despite common narrative from Wayland fanboys, X11 already provides the necessary functionality to implement per-monitor and even per-window scaling with pretty much 99% of the same code that a toolkit would need for Windows 8+ scaling, that is most likely already written? It only needs some cooperation from the window manager and this sort of code could even be implemented for XWayland.... but it needs a bit of modification from the toolkit's side too and while it could be done for Qt5 and perhaps Gtk3, it'd need Gtk2 and especially Gtk1 to be worked on, which wont happen - on the other hand, if Gtk2/3/4 were backwards compatible, all programs would benefit from such new functionality).
Outside of games, for software that touches the Internet you probably also want libraries to get shared security updates.
And of course having to bundle 50+MB of libraries and data (themes, etc needed by a GUI toolkit and any other data by other libraries) for a small executable is a ridiculous idea by itself.
No, a desktop system should provide some minimum functionality that desktop applications can rely on and keep that functionality stable so that application can keep on working.