Gtk3 is awful and broke backwards compatibility with Gtk2 so you need to have at least those two in your system to run applications (since many still need Gtk2). Gtk2 does not get any meaningful updates anymore. Gtk4 will soon be released and repeat the cycle.

Same with Qt5, anything that needs Qt4 or less is now broken unless you use Slackware which tries to keep everything it ever had (it even has Gtk1). Qt5 is made by a company whose income doesn't come from desktop apps anymore and certainly not from being a stable API for a desktop environment. Though even if they wanted, C++ lacking a stable ABI doesn't help much.

The only alternative would be Motif but that hasn't got any meaningful update since the 90s and is only limping in life thanks to a company whose income seem to come from consulting about converting your Motif app to Qt.

Other toolkits either rely on one of the above, have largely been ignored and/or are even less stable than Gtk.

And the majority of all toolkits are way too bloated, not just in terms of resource use but also in terms of how easy they are for a developer to master them.

Neither gtk3 nor qt5 are particularly heavy as far as resources usage. Keeping older versions of libraries like gtk is the cost of keeping a rapidly shrinking cast of existing apps working. Note that qt4 and gtk2 are 7 and 8 years old respectively and can be used with current OS if needed by legacy apps. If your distro opts not to support it the dev or packager can ship it with the application. See flatpak or better nix.

The storage consumed is reasonable in terms of modern sizes storage.

Purely from the perspective of the end user I'm not sure I understand what the difficulty is.

The user doesn't care if the app is gtk 2 or 17. From the user's perspective they click install and an icon appears and their free hard drive space goes down a tiny amount.