JspPage & HttpJspPage interfaces are provided to make request object. Jsp is just a view while servlets doGet & doPost method take request object as a parameter & pass response to jsp page(view).
Ok now think servelt as a responser OR request processor, jsp cant pass request object itself thats why these 2 interfaces are provided, to take jsp parameter & make request object so that servlet can take that & process. HttpJspPage provide service method that process HTTP protocol & all. Servlet interface also provide service method But that decide which method to call either doGet OR doPost.
Your point is Sun can do same this from one interface ,Yes probably Sun can did but it will spoil oops concepts & also will lead to a Bad coding style. I think one of java's feature is abstraction.If they will do like that way then this oops concept will not be followed.
thats all up to my knowledge. Anyway good Question