I agree that trust is part of it, but I don't think it's the whole story. A lot of this is also due to cognitive biases. For example, optimism biases make public works project managers continually underestimate cost and schedule. Then sunk-cost biases make funding managers scramble to find additional money to make up the difference. Often, those funds come in the form of "unspent" funds from other projects.

That's the generous interpretation. There's also the case when project managers deliberately low-ball their project estimates to take advantage of the phenomenon. I'd put that in the trust problem category.