When you go through the funnel, adding a cabin luggage (or anything basically) usually doubles/triples the price.
Most of regular airlines include a proper cabin item up to 8kg.
Furthermore, it is not possible to have a just cabin item option in Ryanair's booking website. It is always bundled with things like priority boarding (read: wait outside & standing longer while other people depart from the plane)
Nevertheless, these practices are misleading. Hurts customers more, creates unnecessary complaints and bureaucracy as well.
Because you can in fact pay a regular airline a reasonable price (40-60eur) and get the standard package (with cabin luggage) and get treated like a human being rather than a farm animal. (Btw adding cabin luggage bumps the ticket price 45-55 eur range in Ryanair's case. In a normal airline, you get a beverage & croissant for free while you must pay 5eur for 0.2L water in a Ryanair flight)