The Trump wall and contractor incentive.

Think about incentives here. Does a construction firm have much incentive to provide a wall that really is un-defeatable? That's a helluva task and an expensive one. The wall pitched to the government only has to be better - that is, appear to government testers to be better - or be almost as good but cheaper than the others that are pitched to them. They also know that cost overruns on public projects are normal, and frequently paid, rather than the contractor bearing the costs for their mistakes. So if mid-project the weaknesses are suddenly "discovered," you already have the contact and can likely increase your earnings by belatedly fixing them.

Repeat business is probably not a big concern here - border walls are not going to go up for bid regularly and forever - nor is public image likely a big concern (quick, name any of the firms that built existing sections of border wall/barriers).

Both of these are in contrast to pitching a bid to private firms. Private firms are concerned about value, not simply price. And if any particular firm is not likely to be a potential repeat customer (and they often are - it depends on the product/service), the contracting firm's public image is likely to always be a concern, because word gets around.