Why do US universities have strict formatting requirements for dissertations?
I think this is a historical issue more than anything else. Until quite recently, dissertations were usually sent for microfilming, microfiching, or both, for later reproduction and distribution. As a result of this, they needed to be carefully formatted so that they would reproduce correctly when photographed. Thus the requirements for font size, margins, line thickness in graphics, positioning of page numbers, and so on.

In recent years, this has not been so important, but that doesn't mean universities have completely relaxed the rules regarding formatting. There is definitely an interest in having a consistent "look and feel" for theses from the same institution.