Windows 10 can run virtually all apps compiled against Windows 2000 just fine, and those apps did not have to bundle their own graphical toolkits. Windows has gone through several new toolkits but they always preserve the old ones so that old programs continue to work.
By contrast GTK has regularly broken things between even minor version updates. Distributions also drop the old major versions of toolkits much more quickly. GTK3 was first released in 2011, but by 2018 most distributions no longer provided GTK2 pre-installed.
Is it any wonder that no one can ship and maintain a binary app that targets GTK without bundling it? Of course bundling sure looks like an attractive solution in this environment, but it's the environment that's the problem.