Whenever you do get pocket aces or pocket kings, your opponents are going to fold when you raise, leaving you with a small pot.
Also, playing only good starting hands makes you incredibly easy to read. This is something that can be easily checked in your database of hands with software like Holdem Manager. In cash games, you will be unknowingly decreasing your winrate by folding too much preflop or it might even turn you into a losing player. Playing only good starting hands in tournaments could mean having your stack gradually eaten away by the antes and blinds as you continue to fold the not so great hands you get. That said, it is not feasible to limit yourself to only the best starting hands because they don’t exactly come around as often as you would like.
