"Er...because it was a hard stone and they wanted hard stones as foundation rubble and these were stones in the vicinity ?"

Hmm, but does it happen with equal frequency at secular buildings?

If you're building foundations your main object is to spread the load evenly, so as to avoid differential settlement and consequent cracking in the superstructure. A clue to the existence of large flat sarsens in church foundations might be cracking in the walls, each side of where they might be.

I think if a builder was going to put sarsens anywhere it would be at the corners. Unfortunately, that's also where you'd put them for ritual reasons too.

I look forward to Littlestone's survey of cracks in churches and whether they follow the same pattern as the known sarsen placements.