This is because the Spider-Man movies are distributed by Sony, who retains the rights from the pre Marvel-Studios deal they made. They do not want to give those very valuable rights up and while they struck a deal with Marvel to co-produce the movies, they retain streaming rights. Now, Sony does not have their own dedicated streaming service, so they could make a deal with Disney, but they could just as easily do so with Netflix or anyone else too. For now we jsut have to wait and see.