Knowest thou not this of old - That is, dost thou not know that this has always happened from the beginning of the world, or that this is the invariable course of events. His purpose is to show that it was the settled arrangement of Providence that the wicked would be overtaken with signal calamity. It was “so” settled that Job ought not to be surprised that it had occurred in “his” case. Zophar goes on to show that though a wicked man might rise high in honor, and obtain great wealth, yet that the fall would certainly come, and he would sink to a depth of degradation corresponding to the former prosperity.
Since man was placed upon earth - Since the creation; that is, it has always been so.