The problem is that they are only temporary jobs.
True to the extent that all jobs have a start and an end, but improving infrastructure should lead to increased private investment as well. For example, nobody is going to expand a factory in an area with old and inadequate water and sewer, or build a distribution center near a crumbling jammed-up highway.
Plus even one-off projects will add to the backlog of companies that have been struggling in the Obama economy and will help keep many from bankruptcy or failure until the Trump economy starts generating more prosperity.