Based on this discussion, it seems obvious that datacenters should only be built on unpopulated land. There'a lot of that in the U.S., in the form of deserts, mountainous regions, and large swaths of Alaska.

What? No, nothing that extreme. You don't need to go into deserts, or into Alaska.

Just build them the same place you would build industrial uses. Nearly all jurisdictions have industrially zoned land, occupied by warehouses and light industrial uses (tile stores and auto body shops and lumber yards and whatnot) and industrial parks. Many also have actual heavy manufacturing. Those industrial areas are usually separated from residential uses, while still being a modest car/bus ride from those areas so the workers can get there. That's where data centers can site just fine.

Barring that, you can take land on the urban fringe - the edge of town - and zone it for data centers. It will cost more to extend the infrastructure out there, but that's probably a capital cost they can absorb.

As for powering datacenters, why not pass legislation to require them to "bring their own power" in the form of clean "green" solutions like solar, wind, geothermal, or small modular nuclear reactors.

Sure. You can tell them they can't hook up to the grid, or that if they do they have to pay the capital costs to expand the grid to accommodate their needs (ie. rather than building their own power plant just for them, they build an extra power plant that goes into the grid). Environmentalists hate that, though, because that extra power plan isn't likely to be solar or wind, but fossil fuels.