13 euros for a plane ticket is an anomaly and probably due to governments funding airlines to encourage tourism to their countries.

Governments subsidizing airlines over rail is one of the criticisms levied in the article:

The group has urged governments on the continent to introduce long-term, affordable “climate tickets” for public transport, including cross-border ones. They suggest these should be funded by a “phase-out of airline subsidies and a fair taxation system based on CO2 emissions.”