The compound : 1-methyl -1- ethenyl, is called as Isopropenyl in common name system. Why is it not called as isosecondary propenyl?

In common system of naming an organic compounds preffixes like- n-(for normal),iso and neo- are used.The prefix iso is used for the molecules containing (CH3)2CH- called as isopropyl group or we can say that when there is one CH3 group is attached to second last C-atom.So prefix iso is already denoting the carbon atom containing the group(substituent).Thats why there is no need to again use the word secondary,as word secondary denotes the carbon atom which is attached to two other carbon atoms.