"Spend per employee" is not a great metric. Most SaaS services allow companies to hire fewer people as they expand, but this metric does not capture that.
"Most"? I don't know about that. Maybe some do but I'd say most are the same software companies used to buy once every three years or some new thing companies need to do (like live chat). I don't think most saas is keeping down headcount, that's giving it way too much credit.