The discount airlines have low fares but have extraordinarily petty charges. Charges to check-in online. Charges for a soda. Charges to carry suitcase onboard.

The traditional carriers, maybe they need to fill seats for those routes on those days (of course if you close the loop, pax carries the other direction needs to be transported back...)

Anyways, i wouldnt make a blanket assumption that intra-EU flights can be easily booked <100