The Flynn effect is a substantial increase in the average IQ over the 100+ years since IQ testing began, especially the Fluid intelligence component. Because of the large change over only a few generations, it is generally supposed that this could not be due to genetic change in the population. This argument (that the effect must be environmentally caused) does seem plausible, though it would be interesting to see someone seriously engage with the possibility of genetic influences, in light of emerging evidence supporting recent (and presumably ongoing) human evolution.
So the effect does demand explanation, and Flynn's general explanation is plausible. (See Heritability estimates: IQ paradox resolved and Beyond the Flynn Effect). The change might be caused by broad changes in the culture (experienced by everyone) which change the mental world, demanding different capabilities. The period spanned by IQ tests was a time of rapid economic and social change. Farm mechanization drastically increased farm productivity, reducing the need for rural labor, at the same time that ongoing industrialization increased the need for urban labor. Before this period, even in “industrial” countries such as the US and England, the majority of the population was still rural agricultural workers. Urbanization brought that bulk of people into a far more complex and abstract material, economic and social environment.
To say the modern world is more complex than that of an 1800's farmer or a prehistoric hunter-gatherer does not entirely capture the nature of the change. People's social worlds have always been complex. Traditional lifeways of tribal peoples are regulated by complex systems of social obligations and taboos. For example, kinship systems often described many more gradations of relationships than we now use. See Vengance Is Ours. Even when you look at the material world, it is not so straightfoward. What is more complex, a ball of mud, or an electronic microchip? The surprising answer is that by usual technical definitions of complexity, the ball of mud is far more complex. That complexity is not fully accessible to human senses, but it is likely that peoples for whom mud was an important structural material and plaything, they will recognize many qualities of mud that we might not.
Likely what is more important about the modern world for the Flynn effect is the increasing use of abstraction. Manufactured objects are actually simpler in the sense that they are more ideal in their structure, having less randomness (Entropy). Artifacts such as a plastic cup or a toy block are far closer to a Platonic ideal than any natural or handcrafted object. In the modern world we also have complex artifacts (such as cars) that are visibly composed of distinct functional parts, and we frequently discuss those parts as separable things.