A football shirt represents more than its colours and design. It can be linked to a specific club, player, season or important moment in football history. That is one of the reasons original shirts are collected and kept for years.
Buying an original also means getting a shirt that was officially produced by the manufacturer for the club or national team. The materials, labels, badges, stitching and other details are made according to the specifications of that particular shirt. A fake one may look similar, but is not an officially produced shirt and the details can differ from the original.
For collectors, that authenticity matters. You are not just buying something that looks like the shirt from a certain season; you are buying the actual official version from that period, as a piece of football history.