For what it's worth, while on any single contract you might get bad behavior from a contractor, their reputation will matter when they go for a recompete or bid on a new contract vehicle.
Not all contracts are automatically won by the lowest-cost bidder. Yes, it allows corruption on the part of the person doing the hiring, but it's better than the alternative.
In California, Tutor Perini seems to be famous for winning government construction contracts despite constant failures and unexpected price increases, because the law does require lowest bidder and doesn't allow ignoring low bids from known liars.
Not all contracts are automatically won by the lowest-cost bidder. Yes, it allows corruption on the part of the person doing the hiring, but it's better than the alternative.