Because some large ships take more than two years to build.
Typically, with such a large project, the purchaser will make a down-payment and sign an agreement to make payments on some schedule typically related to the builder's progress. A builder will want a commitment that the project isn't going to be abandoned before it's completed without some sort of severance payment. Absent such a commitment, a builder is going to want to charge so much up front for the project that once the bulk of the money is received the builder may no longer have much interest in its timely completion unless the government agrees to substantially overpay for the project.
There are strict constructionists, (and I consider myself one of them), and then there are constitutional absolutionists who want to wear 1700s blinders and ignore what the Founders would have clearly permitted had technology advanced to today's levels.
There are a lot of wealth redistribution programs in this country from welfare to farm subsidies that I don't think have any plausible basis on a federal level when looking at the Constitution.
But a constitutional reading that says we should defend ourself against bin Laden by unleashing pirates is just plain looney tunes.