Why are there two "to"s in "from ... to ... to ..."?

It may be a bit too idiomatic instantly to grasp. However:

Animals can live anywhere, from forests to deserts ... and ... from said deserts to deepest pits of the oceans.

The second "from," and that which follows it, is dropped in the name of brevity and verve. Fascinating, isn't it?

You can use as many "to's" as you like, by the way. Like this:

Animals can live anywhere, from forests to deserts to deepest pits of the oceans to mountaintops to cities to other planets so long as there's a bit of oxygen and lunch is served regularly.

Are the forests and deepest pits of the oceans extremities of a range, with deserts being a middle ground?

Typically this is not intended to indicate a middle point between two extremes, but another dimension or member of a category. Deserts, deepest pits of the oceans and forests are three members of a category varying along multiple dimensions, not along a single continuum. The phrase generally gives emphasis to the breadth, of a concept, and how the application of the concept is universal.