Here's a few reasons:
- OSHA and similar: procedures that slow the process to improve safety. They also add head count to the simplest of operations. Not saying it's bad to be safe, but it add humans and cost which reduces productivity
- BIM and similar: Despite promises, BIM adds 30+% to the drafting process and have not yielded reduction in field coordination issues. You can blame lack of training and experience, but the problems continue.
- Increased complexity of materials, designs, and methods: newer materials require experts who in short supply and getting access to them introduces delays. Plenty of rework when things are installed improperly. Typically when it's more disruptive and costly to do so.
- Sprawl of technology throughout buildings: typically designed by people who bare know how it works and installed by people who know nothing about it = mistakes, rework, additional purchases, etc.
- "Agile" mindset of the owners/occupants. So much of our lives is software driven and can be rapidly changed. Construction isn't, but the people paying for it think that way and operate that way. Many late and disruptive changes lower productivity. It's always interesting to watch newer tech companies or even non-tech companies that run some form of agile for operations try to handle a construction project.
- Complexity of solution: 8 parties are now required to coordinate where 2 may have in the past. Challenging for design, but a tremendous headache when building and troubleshooting. All requiring more resources to be engaged longer for seemingly the same unit of output
- LEED, WELL, sustainability: Introduces new design reviews, re-design, new materials, new processes, etc. See rants above for how this reduces productivity.
- Security: job security as well as physical and info security. Just getting on a job site is difficult and slow. new physical security measures add infrastructure and complexity, and this group is aware of how infosecurity makes collaboration across the many different contractors and consultants slow and difficult.
- Compliance: for one client, I've had 60+ hours of compliance training because I need to use their systems.
In most of the above, the result is a better, more capable product that was built more safely with a reduced environmental impact, but productivity suffers as a result of those gains.