When creating a movie, a lot of factors come into play. In general, animation and live action film production may produce similar films and stories, but ultimately the creative process is vastly different between the two.
The argument
Producing animated films is entirely different from producing live action films, and proponents of this position argue that the difference is large enough that the two cannot sensibly be judged against a single standard. The biggest difference is where the work happens. An animated film can be made by people all over the world; it isn't necessary for actors, directors, and writers to all be together in the same space. There are no real sets when it comes to animated films, and different parts of the film can be made simultaneously rather than in sequence. A live-action production is bound to a place and a schedule, since the people, the sets and the light all have to coincide, while an animated one can be distributed across studios and assembled afterwards. The craft differs as much as the logistics do. In live action, continuity between scenes and shots is essential, and much of the discipline of the form consists in maintaining it. In animation there is a lot less importance placed on this aspect, and animation style may purposefully differ from scene to scene for storytelling and tonal purposes. That freedom is a genuine expressive resource, and it is a lot harder to accomplish when it comes to live-action films — which means the two forms are not doing the same job by different means, but pursuing different possibilities altogether. If that is right, a single award cannot rank them without arbitrarily privileging the conventions of one of them. Allowing animated films to be compared to live action films is essentially encouraging the comparison of apples and oranges. Thus, there should be two different categories for Best Picture, one for animated films and one for live action films, so that each is judged against work that faced the same problems.
Premises
Counter-arguments
Yes, the production of animated films differs greatly from the production of live action films. However, the end product, the films themselves, are similar enough to warrant comparisons. If the Academy is awarding Best Picture to the film that was most effective at telling its story, then it is reasonable to compare animated films to live action films.
Rejecting the premises
[Rejecting P2] The films themselves are similar enough in the way they tell a story, and thus are comparable.