This is not the claim.
The word “objective” does a lot of important heavy lifting here, and there are strong arguments to be made that subjective experience has gotten significantly worse, while most “objective” factors have continued to improve as the world modernizes.
The trouble is that evolutionary psychology is such that the major changes to how humans live and communicate leave us in an environment that looks less and less like the environment we evolved in.
This is why there is so much interest in the impact of social media. There is evidence that even among those who are in an objectively “good” circumstance when defining “good” based on safety, financial security and other commons measures, individuals are just as susceptible to the negativity and distorted reality that dominates modern social life.