Of course it doesn't make sense to give refunds. If it did, Ryanair would already do it.
Very few people go through the trouble of getting a refund for airport taxes, which means that the revenue from no shows goes almost directly to the bottom line, as there are very few costs involved in it for Ryanair.
Ryanair, like any other large airline, has algorithms that predict the number of no shows on any given flight. Sure, ithey get it wrong occasionally, but by and large it's a numbers game.
Ancillary revenue has costs attached, which no shows don't. As for encouraging further bookings with refunds, people will not think twice about booking their next 10-20 € fare, no matter the lack of refund.