Perhaps this adds little to the answer already posted, but I feel it needs to be emphasized that
airliners are designed to fly.
Designed to fly a lot, in fact. Like, spend a significant portion of their working lives in the air. Southwest Airlines, for example, has each of its 737s in the sky for 9 hours a day. I imagine Ryanair's number is similar. And most of that time on the ground is spent moving as quickly as possible to do the tasks required to get back into the air again. Basically, having your fleet sit around is an off-design condition.
Not only is it an off-design condition for the aircraft itself, it's an off-design condition for the entire maintenance concept and infrastructure of the airline. An airline knows how to put airplanes in the air and keep them there as much as possible, because that's how to make the most money. (If you're an airline, of course; aviation is generally not the way to make the most money.) Airlines are not experts at parking large numbers of big airplanes for a long period of time and then returning them all to service. In fact, no one is because, in general, this situation just doesn't happen.
So, your choice is either go through the process of mothballing your airplanes for an indeterminate period, which involves a bunch of maintenance tasks you're not equipped for (not to mention finding a place to put them) or that you'll have to pay someone else to do (likely at a premium right now), or try to keep the fleet operating as close to the design condition as is reasonable. The former option has the added drawback that the airplanes are likely to be a bit cranky for a while (i.e., more unexpected problems) after coming out of storage. The latter option has the added benefit of keeping flight and maintenance crews more current.
We can even run some rough numbers. Based on the article, let's say each airplane currently flies about an hour a day rather than nine. That means it might be due for what amounts to an A check every year or so, which at the high end could take 100 maintenance hours performing tasks that the airline does all the time and has the people and materials in place for. Compare to the process of mothballing and then unmothballing an airplane, which might take at the low end 200 hours of tasks that are not often performed. Of course in the interim the cost of flying is greater than the cost of sitting, but still it could be cheaper for an airline to run these "ghost" flights for quite a while than put their fleet in storage.
Overall, it is simply a less financially risky proposition at this point to give the airplanes a little bit of time in the air every once in a while than ground them and expect to resume service quickly. Flying and maintaining them "as usual" presents a known situation for an airline, especially when the horizon for returning to normal is closer to months than to years. Storing the aircraft for what could potentially be a relatively short period (as far as these things go) is fraught with unknowns.