We have all the spend data — why can't we find the savings?

You already have the numbers. That was never the hard part. The hard part is that a spreadsheet does not tell you what to do — it only tells you what happened. You are treating a decision problem as if it were a visibility problem, and no amount of staring at the same columns together will fix that. This is also, quietly, a comfort problem. It is easier to say 'we need better reporting' than to say 'I looked at category X and it is bloated and I did not act.' A group can hide in this together longer than one person can alone — everyone waiting for someone else to name the cut. You do not need more data. You need someone to own each category and answer one question: what would you cut if your own money were on the line. Then you need to actually cut it.