Oh, it is not that software like freecad is not parametric by necessity; it is just that in a parametric design process, you don't set all the parameters at once. Why? Because a) you don't overconstraint a design, and b) your parameters rely on references (a point, a line, the side of another object, a tangent), and you may want to change which reference you choose.

For example, if you put a hole in a box, do you want to reference the center of the sides of the hole? And do you want to place them relative to the left side or right side, and front or back? You never say "it's x mm from the left and y mm from the right and my box is w mm wide" - because the relationship x+y=w is always here! You only define 2 out of 3. But it may happen that you picked the wrong 2 - and a parametric CAD makes it very easy to do so in a few clicks, while a programmatic CAD like OpenSCAD is a large rewrite with calculation you have to do yourself on the side.

My example is silly but start doing big designs, large assemblies, and you "chains of cotations" may need to be redone again and again - especially when tolerances begin to add up.