Because to simply achieve means simply average. We are raised to believe that there is something wrong with being average; you have to have that "A", you have to be the best, you have to do more. We are told, directly or indirectly, that we have to prove ourselves to be worth something. Are we trying to prove ourselves to other people or to ourselves? Is it because we need to beat that person that is always one step ahead of. Except, they're always going to be one step ahead of us, and someone else is always going to be one step ahead of them somehow. Or if we do manage to get ahead of that someone, then we have to get ahead of the person that is ahead of them. In the end we are constantly trying to get ahead of someone else.
But when it's all said and done, what worth is there when you are run ragged and have no enjoyment of the simple pleasures of life. We're left with a successful career and an empty house.
The balance of these two ideals takes very careful thought, while maintaining the flexibility to change. A successful career means accepting that you aren't the best. A successful house means making time for it. Time cannot be found, only allotted. Allot the time for life, or just watch yourself be carried away.