Economics explained YouTube channel has a good coverage on the rationale behind the crazy projects like this. In short, while the project itself might not make much sense, the infrastructure created to build it will stay and it makes it more accommodating to build other, more rational projects.
That's a rationalization, not a rationale. Every project can have useful byproducts, and nobody's rationale is to start a dumb one trillion dollar project rather than a smart one.
The reason why this exists is obviously that these projects are started by absolute rulers who want to build themselves vanity monuments the same way their ancient predecessors did.
Maybe I'm cynical but I really doubt that the Crown Prince is smart enough to have thought about it like this and even then, they could achieve the same (supposed) end result while also pursuing a more achievable mega project and get more out of that $1.5T.
If you're only building a mega project so you're forced to develop the infrastructure around it, why don't you choose a sensible megaproject instead? Answer: because that's not why they're doing it, you just made that up.
https://youtu.be/pnrAxfzHElo?si=ceaS6MIs7qrP9ohL