Am also a design engineer, I specialize in water and wastewater works. My clients are all municipalities, with tight budget and politics are a factor. Engineering productivity has climbed thanks to computers, but construction productivity continues to decline. In my experience on small jobs this has a lot to do with safety and regulations. It takes a team of 2-3 to enter confined spaces (manholes) for momentary inspections or maintenance, it takes extra workers to set up traffic zones to ensure travellers are less of a danger to the workers. Time is taken to ensure archeological, agricultural, and culturally sensitive areas are not disturbed. Minority and women owned businesses are given contractual preference, whether they are most qualified or not. It takes a special (read: expensive) team several weeks to document trivial wetland areas (most people call them roadside ditches), and another person weeks of labor to explain how impacts will be minimized. The government sets standard labor rates for construction labor.

But these are things we as a society have deemed important. Its not acceptable for lives to be lost. It's not acceptable for construction workers to accept low wages. It's not acceptable to recklessly degrade our environmental resources, and it's important to have diversity in this industry.

I don't know if it's true in other countries, but it seems the USA vascilates between priorities depending on the public administration. I am young so my experience is short. Bush saw a real estate bubble, Obama saw an insurance bubble, Trump et al aim for a construction boom. I would add that in New York my home state, a Democrat state, there is a large infrastructure program starting, so it's not just Republicans.