You don't actually need a passport to travel within the EU. Just a European id card. Still, most people I know do have passports, but that might tell you more about my particular bubble than anything else.
No point in an ID card when a passport is only slightly dearer, but more useful.
The form factor. My national ID is the size of a credit card, so I always keep it in my wallet. For something that lets me visit a good thirty countries, that's good enough value for me.