Because there is little to no correlation between a fund's past performance and a fund's performance in the future. Chasing past winners actually makes you LESS likely to beat the market average! (I can provide pointers to academic studies if you like)

The past ten years were the most amazing bull market in history. Now times have changed, and it's not nearly as easy to make money. Strategies that performed extremely well then might perform extremely poorly now.

If you're investing for the long-term and don't care about month-to-month (or year-to-year) fluctuations, indeed you should be aggressive. But don't take unwise and unnecessary risks.