Is there any reason I shouldn't make my own stair treads?
Wear.

The overlays appear to be solid hardwood.  The plywood is a thin veneer over other non hardwood plys. As the steps wear, you run the risk of wearing through the veneer. Even if you don't, if you need to refinish, you again risk sanding through that thin top layer.

In my old house, the bottom stair had a worn dip in the middle (reputedly eroded by my younger son's butt that perched there in his many time-outs). A veneer would have been long gone.