There is no "Biblical" support for an allegorical interpretation of the Book of Genesis.

Rather, it's a retrofitted philosophical stance used to explain events in the Book of Genesis that may or may not have happened exactly as we previously thought.

Based on our own human Naturalist explanations and discoveries of current methods of science, archaeology, Biblical scholarship etc.

That seem to contradict with a literal reading of the Book of Genesis.

Or more broadly, several other Holy Books of the Bible that are contested for their historicity.

It's also a method of trying to grapple with the a palpable answer for the Problem of Evil.

It's in all honesty, an apologetic tool. Albeit one that can be misguided, especially when not explained appropriately onto why you are using it to defend Christianity.

As for myself, I take all Books of Holy Scripture, just like the Book of Genesis to be exact.

As true, literal history, and events that did indeed happen.

HOWEVER, I am also open-minded to entertain an allegorical reading of any Book of the Holy Bible, if the they can be proven empirically, without a shadow of a doubt untrue.

As I cannot, nor ever will accept that Christianity is incorrect.

I hope this explains the concept a little more.

In the end, you will find not a shred of Biblical evidence that an allegorical reading of any Book is to be presumed.

It only stems from sheer philosophy and nothing more.