People can work hard using all their wisdom, knowledge, and skill, but they will die, and other people will get the things for which they worked. They did not do the work, but they will get everything. This is also unfair and useless.
If people please God, God will give them wisdom, knowledge, and joy. But sinners will get only the work of gathering and storing wealth that they will have to give to the ones who please God. So all their work is useless, like chasing the wind.
Do not be like Cain who belonged to the Evil One and killed his brother. And why did he kill him? Because the things Cain did were evil, and the things his brother did were good.