We all need more vicāra
The list of the five jhāna factors starts with connecting and sustaining attention (vitakka and vicāra). Connecting is not difficult, because our attention spans still have a few seconds of qi in them, but sustaining is our mortal weakness. We mostly can't do it. Distraction is a symptom of failure to sustain attention.
The real key to jhāna can be found not deep in the Ānāpānasati instructions, where we're attempting to calm and gladden the subtle mind, but in the preliminary instruction, the one that precedes even the long and short breathing: "mindful one breathes in... and out." That's it. But we can't do it.
If you can figure out how to be with the cycle of the changing breath without looking away, the door opens. It's almost neither subtle nor spiritual, just a straightforward way of being focused and intent that may once have been commonplace but now seems almost supernatural.
The training in meditation is about vicāra (sustaining focus), almost exclusively, for far longer than we think. Everything else we do—all the beautiful mettā, vipassanā, and wisdom reflection—is either taking the place of preliminaries because sustain was hard to find on its own, or advanced practices that shouldn't be as difficult as they are. They're difficult because we don't have sustain. Sustain = sustenance.
