Yes, they want to put these things near existing power infrastructure (ie cities), but per NIMBYs they cannot. To the extent they can use existing power infrastructure, they are using existing power infrastructure and then only adding what they need. In lieu of a significant bust, this is far more likely to be a negative externality than a positive one.
Of course people realize this, so they do not want to build data centers nearby, ergo data centers are increasingly likely to be sited "in bumfuck nowhere," even up to and including literal outerspace.
Here, the positive externalities are basically zilch regardless of bust or not.
I get the 30,000 foot view that you are describing, but I am saying specifically in this scenario, for this infrastructure, in the specific existing political/economic/land use context we are talking about, I don't see the argument.
Past infrastructure booms have built infrastructure in and around cities. You have just described why that is increasingly not the case, and even if it were the case, it's only a positive externality if there's a giant bust, which feeds back into people wanting to keep these things away from cities!
Please identify a few of the specific infrastructure booms that you think are analogous.
Dams: nope.
Telecom: nope.
Broadband: nope.
Electrification: nope.
Fiber: nope.
None of these look like "build infrastructure in middle of nowhere where if the project succeeds, it consumes everything it created, and if it fails, there's nothing very useful for others to consume that infrastructure in those locations."