The way I see is that it is for the same reason that Qt and GTK include all the widgets and abstractions to create a complete UI.

Another framework that I've used in the past for 2 WebApps projects[1][2] -and I was quite happy with the result- is Semantic-UI (and the fomantic fork)[0]

[0] https://github.com/Semantic-Org/Semantic-UI https://fomantic-ui.com/

[1] https://github.com/sighupio/gatekeeper-policy-manager/tree/v... [2] https://github.com/ralgozino/vmtory#screenshots