An emulsion like mayonnaise is not a pure fat, though. It’s a mixture of different fats (both saturated and poly-unsaturated) and water. The mixture is stabilized by emulsifying agents like lecithin that interact with both the fats and the water. When all theses ingredients are blended together with enough force they create an arrangement that is semi-solid at room temperature, yes. But you can’t compare that fairly to a pure fat. If you try, you’re missing the point.

That said, mayo is a great replacement for butter in some situations. Try spreading a little on bread before toasting it in a hot skillet.