Why can't you buy a seat for an infant on Ryanair.com?

Question

We tried to buy a seat for our baby on Ryanair.com and it was impossible. We are willing to pay same price for the baby as for our tickets. I understand that they allow babies to fly free on your lap, but if you want to pay to book a seat for an infant you should be allowed to do so.

Answer 1

Because an infant would have to sit in a car seat/baby seat, and those are not allowed on the aircraft.