With the state of the music industry now, maybe U2 are turned off.
Perhaps the days of making albums are not worthwhile for them anymore.
We should be considered "lucky" that U2 have released albums at a sort of regular pace - although that has slowly diminished over the years. Some bands - like the Rolling Stones, or The Eagles, or even Van Halen (14 years between albums?), don't release albums at a regular pace. So it entirely possible that U2 may just decide to call it a day and release their next album many years down the line.
We should consider ourselves lucky and privileged. Some fans in these forums seem impatient and "entitled" to have the next album out sooner than later. Then they seem to get mad when Bono does his usual schtick of the random media soundbyte saying what they are currently recording is their best work yet and they can't wait to put it out, or they make fun of Bono's hyperbole.
To your point, U2 are surely reaping rewards from their previous tours but that is not a reason U2 are not releasing a new album yet. U2 are also reaping awards through royalties and sales of their back catalog. Their cashflow is always there and is not limited to 360.
Recording an album involves costs/expenses (hiring producers, sound engineers) and we all know U2's are more costly than other bands - as they work with multiple producers, ditch them mid-project, then they record across multiple continents in different studios - and their process has no set plan -- they just jam until the music comes in which is know to drive their staff crazy and make it take forever to record albums. With the state of the music industry now, perhaps they are studying if they can still profit or keep their legacy without cheapening their image.