Why aren't more retailers offering customization?

As I see the success of customization at retailers such as Build-A-Bear, I wonder why other retailers haven't engaged in more extensive efforts to provide customization experiences in their stores. With the web disrupting brick-and-mortar retail, I've argued on this blog that retailers need to get more experiential. Customization serves as critical component of an engaging retail experience. Certainly, one can customize over the web too. However, in some cases, brick-and-mortar stores can provide a more satisfying experience, in which one examines and chooses from various options and builds their own product.