Maintaining the capability of building actual, physical things is important in many ways. probably the most important is the well-established synergy between manufacturing and R&D. This is because when you're the one actually making something, you have an incentive to find new and better ways to build it, plus an incentive to improve upon what you're building, plus an incentive to gain the knowledge that allows you to understand that what you're building may eventually be replaced, hopefully with something better, that hopefully you'll have learned how to design and make. When somebody else is doing the building, they have those incentives, and you probably don't.
The other reason is that we end up placing our fates in the hands of outsiders. And there may come a time when, for reasons other than those found in the marketplace, those outsiders may be less inclined to want to supply them to us, no matter what tribute we may be willing to pay them. (We had a taste of this relatively recently, in fact, if you're old enough to remember the OPEC embargo of the 1970s.) That may be okay as long as what you're lacking is not vital to maintaining or defending the country. But if it is, you're probably going to be in a world of $hit.