why must we tear up undeveloped areas for data centers instead of backfilling vacant industrial areas? Humanity will never rest until all of the world is a brownfield

It's cheaper to build on vacant land than to get construction equipment and materials into a built up area, possibly discovering underground tanks and other surprises that can inflate the project cost, etc. City ordinances may also limit work hours and/or noise.

Rural land is cheap and there are fewer neighbors to annoy with the 24/7 construction activity.

The current NIMBY trend with respect to DCs guarantees it.