Normally when boarding ceilings you screw to the joist lines, not the solid strutting. When you have a board balanced on your head with maybe the other end supported on a tee it's kind of difficult (often impossible) to hit a piece of solid strutting which runs at right angles to the joists - it's far, far easier to aim for the joists, because you can see where they are located down one edge of the board, and on the other edge you'll either have pencil marks on the wall (first row) or a nice row of visible screw heads and you can assess where the joist line us quite easily. It would waste a lot of time messing about measuring where strutting was so you could screw into it.
If you'd ever seen a pro installing a ceiling, or even installed one yourself, this would be obvious to you. You might conceivably get a few screws where a piece of solid strutting is visible on a board edge and could be screwed into, but I can't see a pro doing that very often, either