Try building Inkscape or GIMP from source on macOS and see how "multi-platform" those GTK apps feel in practice. Even when a Mac build exists, it is often skinned oddly or lags because somebody has to carry Mac patches against an old fork.

This is for the long tail. The compositor path dodges a pile of volunteer-port churn and runs the Linux build directly, which is a lot more appealing for niche GUI tools and dev apps that barely get maintained on Linux, never mind macOS.