Earlier quoted context omitted.
Ryanair wants to make it harder for consumer to compare prices.
And Ryanair wants to upsell you during the buying process, which they can't do as well, if you don't buy on their site.
Nothing is stopping airlines from offering 1-click bookings, except that lengthy funnels bring in tons of cash. Ryanair has been a master at this since the beginning of time.
In 2020 I booked a flight directly on Google Flights and the booking was complete before I even realized, it was so simple. I never found that UI again.