At least part of problem is that Saudi Arabia is reaching limits of various types. One of them is inadequate water for a rising population. Adding desalination plants adds huge costs and huge energy usage, but does not increase the standards of living of citizens. Instead, adding desalination plants simply allows the country to pump less water from its depleting aquifers.
To some extent, the same situation occurs in oil and gas fields. Expensive investment is required, but it is doubtful that there is an increase in capacity that is proportional to its cost. To a significant extent, new investment simply offsets a decline in production elsewhere, so maintains the status quo. It is expensive, but adds little to what gets measured as GDP.
The world outside of Saudi Arabia is now running into an investment sinkhole issue as well. This takes several forms: water limits that require deeper wells or desalination plants; oil and gas limits that require more expensive forms of extraction; and pollution limits requiring expensive adjustments to automobiles or to power plants.There are plenty of comments that could be made, but again, I'm too far behind to do much more than just post the link.