Because that is where the existing roads, power and internet connections are. Might not be that significant investment to build those in context of scale. But still it simplifies many steps in building.

They need to be near a high bandwidth fiber connection, but with relatively cheap land. So the outer edges of major cities are prime locations, with Ashburn VA being the prime example. There is no reason to put them further out because the added costs don't make sense and the data centers actually have minimal impact on the surrounding population outside of hyperventilating conspiracy theorists, Ashburn VA also being the prime example.

One of the reasons to build them near populated areas is that it makes it easier to repurpose waste heat to, for example, heat houses in colder climates.

> don't understand why these new data centres are all in populated areas

Power connections.

Probably so they have no problems getting people who work there and provide technical services. I mean that seriously without any sarcasm or tongue-in-cheek, it seems the most likely explanation to me.

They need drinking water apparently.

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