How would that work for deprecations and new API introductions? GNOME is built on top of glib/gobject/gtk and a host of other libraries which all have to be ABI-compatible in some way. And GNOME itself is used as a base platform for applications.
Having a predictable versioning scheme helps limit the number of releases for downstream application maintenance work. You can't expect applications to have the full git tree available at build or runtime to make some features conditionally available.