Because for 2,000 years Christian referred to themselves as being part of the 'universal church'. Catholic means universal, as apart from the breakaway sects. There were a few here and there until Luther.

We still know we are part of the Universal Church and not part of a break away sect from the 2,000 year constant communion of Christians with the direct approval, touch and communion with the Apostles and those they charged with continuing the faith.

The generic term was a novel thing chosen by the break away protestant sect in order to separate themselves from the Universal Church.