I don't make my own soaps either, but I think that a common reason for those who do is that it can be cheaper to make your own. Also, you can be assured that the ingredients in your soap are safer than those used in store-bought ones.
I think it might be similar to mine with lotions or anything else really for that matter, we know what is going into the product. There is no need to second guess "what if" or "can this be damaging to me". At least that is what I get from it.
For me, the attraction to making soaps and other bath care products is that I know what's going in them. Even many of the so-called pure products you buy, are not as pure as they make it sound. Most of today's commercial products have toxins of some kind in them and I want to avoid that as much as I can for me and my family.
Soapmaking is very artistic. It can also be fun and relaxing to make soap and there are so many recipes out there. Commercial soaps are made of detergent and surfactants, plus the glycerin is extracted from it. The glycerin stays in soap and has more skin loving properties. If you know how to make soap you can make many other DIY beauty products out there too that can be really expensive to buy. Many women spend a ton of money each year on bath and body stuff when they can actually make it themselves and give as gifts.