Because research on this topic supports it. Happiness and wealth are correlated.
This is pretty intuitive. Its nice not to have to worry about money, but what is the difference between having 1M NW and 100M? If you're a mentally normal person, it just more mental burden.
And also. Up to a certain point is still a correlation. Getting a lot of downvotes by people not knowing what a correlation is.
People are happy when they are secure and unhappy when they are insecure. Who can you name is secure in all of their physical, social, mental, spiritual, etc needs right now?