Why do they simply watch the tire?

The entire film is a commentary on cinema itself. There are a few different metaphors or interpretations I have heard about the film.

• The film is a criticism of the audience. Everyone who went to see the film paid to watch a movie about a serial killing tire, and they are still critiquing it. Even the audience in the film is clueless about what they paid to come see, but they paid to come anyway. The audience in the film also represents the average filmgoing audience demographically; people of different ages, races, men, women.

• People are dumb. We are told to accept things for no reason, even though a reason can be found behind nearly everything. Illustrated by the opening monologue of an authority figure telling the audience this very point.

• The film is taking a jab at the film industry as a whole. The film industry produces mostly films that are predictable and not thought provoking. People pay to be entertained with dumbed down, garbage because that's what they're told to like. Producers want to hit every demographic and make money, not make thought provoking, meaningful films.

There are plenty of other correlations you can make with the film.

• The audience in the film are given poison, much like the fatty, unhealthy foods peddled at movie theaters.

• Shiela seems to represent foreign, or high-brow film. As she is the only foreign character in the movie.

• The tricycle represents, re-boots and remakes.

I'm not sure if this answered your question, I hope so.