This is the paradox I cannot get my head around. The tasks on IQ tests look almost insultingly simple from the outside. Repeat these numbers backwards. Which shape completes the pattern. What does this word mean. None of those things look anything like the complex demands of a high performing career or a difficult academic program. So why do performance on these seemingly trivial tasks predict real world outcomes as well as they do?
The simplicity is the point and once you understand why, the predictive power makes complete sense. The tasks are deliberately stripped of real world content to isolate the underlying cognitive process without giving an advantage to people with specific domain knowledge or educational background. What looks like a simple pattern completion task is actually measuring how efficiently your brain processes novel information and identifies abstract relationships, which is exactly the cognitive demand that shows up in disguised form across almost every complex real world challenge. The reason digit span predicts outcomes is not because remembering numbers backwards is useful, it is because working memory capacity underlies a huge range of cognitive demands from following complex instructions to holding multiple variables in mind while solving a problem 
The tasks look trivial the same way a blood pressure cuff looks trivial 
The simplicity of the instrument does not reflect the complexity of what it is detecting. A simple repeatable measurement pointing at something fundamental is exactly what good psychometrics looks like 