If you need to see all the schedule options from a given city, simply download the PDF schedule for that city -- it shows all the available itineraries both to and from all the cities served from that city.
If you want to fly on Ryanair from, say, Dublin to Treviso, how exactly does their system make it any easier for you? In general, people want to travel between two specific places, and not just to where an airline takes you.
While not offering connecting itineraries indeed reduces the airline's costs, it also reduces the attractiveness of the product to some passengers. Ryanair doesn't do me much good if I want to travel from Pisa to anywhere but the four cities with non-stops. And while they do offer excellent loss-leader fares, 120 GBP from DUB
in the higher/highest fare buckets is far from low-cost for a one hour flight.