They work better that way. The solo films can focus on the big characters (Stark, Cap, Thor), new characters that go either into franchise (Guardians) or a one-off just as introductions (probably Ant-Man), but not every supporting character needs their own film. That's kind of how I see the Hulk now, and that's cool. That means when he does show up, in an Avengers or something, it'll be much cooler and less 'forced'.