Why is 'oldest university' such a contested title?

Because the definition decides the winner. Bologna, founded 1088, is the usual answer on the narrow European definition - a self-governing corporation of scholars granting degrees. Al-Qarawiyyin in Fez has been teaching since 859 and became degree-granting in the modern sense only in 1963. Al-Azhar in Cairo dates from 970. Nalanda in India was a vast monastic university from the fifth century and was destroyed.

Each claim is defensible and each rests on a different criterion: continuity, teaching function, legal charter, degree-granting, or self-governance. UNESCO and Guinness have recognised al-Qarawiyyin; European academic tradition takes Bologna; and there is no neutral standard because the institution called a university is a specifically European legal form that was later exported.

The interesting question underneath is what a university is for. Bologna began as a students' guild that hired and could fire its professors - students controlled the curriculum and fined lecturers for starting late or skipping material. Paris organised the other way, as a guild of masters. Almost every university in the world today descends from one model or the other, and the students' version lost.