Because it's how they make money. Why do you think they're able to charge £11.98 for a return flight? Their business model is designed around cheap flights and charging for everything you can. And you can bet some people won't be taking hand luggage if they have to pay for it.
You do realise that Ryan Air's seating algorithm deliberately moves people who checked in at the same time further away from each other. Thus compared to other airlines you have to pay far more times for seat allocation. Also surely nearly everyone takes a bag in the cabin? so why charge for it?
Why do you think it's a joke? That's how the airline makes money. Do you expect them to pay for you to fly abroad?
Map of the region.