Even if your soap is expensive, you cannot carve it if it is dry, so when you purchase the bars of soap, keep them in a humid place right away. More precisely, place your soap in plastic storage containers or plastic zipper bags for food with a little water to prevent your soaps from drying out. You do not need to open the package the soap. Moreover, if it is made out of foil and is tightly closed, such as Ichigo Milk Soap, it will stay fresh even longer.
As long as you keep the soap bars from drying out, they will stay soft for more than several years.
The reason why the soap bars that you bought are dry and hard is the moisture in them which evaporated even though they have been packaged in plastic.
See this page if you want restore already dried out soap with microwave.
See this page for the same instructions but without microwave.