It isn't necessary in the first place either. For instance, there's an expression äta eller ätas, lit. eat or be eaten where we use two infinitives without att. Instead, the reason is literary. The classical Swedish translation of Shakespeare was made by Carl August Hagberg (all of his Shakespeare translations available for free here).
Shakespeare wrote in blank verse, which means five iambs (an iamb is like padam) plus often one unstressed syllable, like this (stressed syllables bold):
To be or not to be that is the question
In fact, Hagberg's translation doesn't really fit this scheme, because it contains six iambs:
Att vara eller inte vara det är frågan
But at least it's iambic. And this is the reason why it needs to have an att before the first vara: because otherwise it wouldn't fit in the verse scheme!