Because you do not know what sector might do better or worse. For example investing in an index fund for US stocks may miss the natural rotation to international stocks which may outperform in the future. Or a pharmaceutical index fund may be hot for years but then do really bad. The Callan Periodic Table shows the really well. https://www.bogleheads.org/wiki/Callan_periodic_table_of_investment_returns

Now if you are talking in investing only in VT or VTI for total US or total world stocks that may cover most of your bases - but there have been years when bonds outperformed stocks too. Diversification helps prevent you from performance chasing where you keep rotating out of a sector always hoping to catch the next rising star.