Because the use of the export columns may vary by product type. For example, if a simple product has three attributes, all three attributes will likely have values for that product, and each product will have a unique name (i.e., Title). But if the product type is variable, the variations may share the same product name. Also, if only a subset of the attributes are used to generate variations, then only those attributes will likely have values. These differences may be confusing if you can't see the product type.