Buildings use the strongest material for the $ because weight is not an issue. For airplanes the weight is as much an issue as $ or strength so they use aluminum (or composites).

And as Phoeonix says, it would also be easier to push around space.

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Given the inherent flaws in the design and the shoddiness of the construction, the premiums to insure the DEATH STAR would have been, if you'll pardon the pun, astronomical.

First of all, there's the construction of the DEATH STAR. This is a large capital project. It took twenty years to build. Now, we know that there is a large bureaucracy to contend with (it's mentioned at the beginning of STAR WARS); this means that there are a ton of regulations - many of which are nearly impossible to comply with and exist only to facilitate litigation. A fifty-floor office building can be expected to generate a couple dozen claims. An armored space station with enough firepower to destroy an entire planet which is large enough to be mistaken as a moon would probably create several million claims.

Over the two decades' time, the cost of insurance would escalate ever higher. Darth Vader's sad devotion to the Force is no match for an army of actuaries.

On top of that, there are unions to contend with - plumbers, tin-knockers, electricians, carpenters, janitorial, iron workers, etc. If there's a trade, they'd have a union shop there. And that means escalation of costs in accordance with contract schedules. Plus, they'd pool their power to get rid of mandatory furloughs. We're look at $156 septillion in fringe benefits alone.

As stated before, there's bureaucracy. The Old Republic is going to need an army of lawyers just to deal with the bureaucrats to keep the permits in line. Then there's inspections, contract compliance, bringing in subs and IC's, making sure that the workers' comp premiums are paid and that there's coverage. The legal department would submit a bill that would make the Emperor blush.

So now the Old Republic is deeply in debt and on the verge of economic collapse. And this is just to build the DEATH STAR. Given the number of accidents that happen on any given day in a metropolitan area, the Empire would die a slow death at the hands of claims adjusters, personal injury attorneys, and pencil pushers.

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Also, you have to realize that legal proceedings may be swayed a bit by the dark side of the Force, since both Lord Sidious and Lord Vader can pretty much influence any weak-minded Senate to go in their favor.

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Second, do not underestimate the power of lobbying. Who needs dark lightning when you have K Street?

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Lord Vader's devotion to the Force isn't just a cultural thing. It's also been something of a cash cow.

Sith were prescient enough to patent the hell out of the dark side of the Force, with significant claims to shared components for general use (along with multitouch).

While the Jedi's use of force is well known, less well known are the steep licensing fees they're compelled to pay for the privilege. This information is not widely circulated out of concern that knowledge that the Jedi helped fund weapons used against them would be bad for morale. However, as most Jedi who've mastered the Force can tell you, it's somewhat addictive and frankly, a helluva lot of fun. The Sith knew what they were doing. Over time, the fees began to add up and suddenly building things like Death Stars and Clone Armies were not out of the question, thanks in large part to Sith devotion to the very Force you denigrate, and a willingness to aggressively protect its IP.

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