There are some issues when painting leather that you don't run into when painting other surfaces. First, its's always flexible. There are not many things painted that are always flexing. That means you have to have exactly the right paint, and the right amount of flex agent, then apply only a thin layer. I have no idea who supplies that. Next is surface prep. All paint needs to be able to "bite" into something, That means sanding down the leather, which almost no one does. Finally, this has to be done only on finished leather. If you do it on anilines hides the oil and was infusion from the tannery will never let the paint adhere.

Too much work, too sketchy on the outcome.