For many obvious reasons: available time, available routings, personal priorities. And until recently (and perhaps still in some places), the fact that you can move freely within Schengen now doesn't mean you have guaranteed full access to all facilities unless you achieve the use of a local covid-pass app. Most reports seem to indicate that most venues even in difficult countries are not troubled by an Australian-issued international Covid certificate, but the official rules aren't always accepting it, so you could occasionally still have problems.
