There would be two reasons to not buy right now.
(1) the market seems to be in a state of free fall. You can buy Apple for half what it was selling for several months ago. But next week, you might be able to buy it for half that.
(2) In the case of Warren Buffet, who is no spring chicken, it's possible that he will die before the shares he has bought advance to an acceptable high. I doubt this worries Buffet much.