To be fair, so does elementaryOS. They really try to encourage developers to adopt their HIG.[1]
So much so that most developers using or targeting elementaryOS integrate their design really well.[2] For those that don't but still use GTK3, they also integrate quite well out of the box.
[1] - https://elementary.io/docs/human-interface-guidelines
[2] - https://quassy.github.io/elementary-apps/apps/