Because “has dropped 20%” is a statement of the past. “Are” in a bear market is a statement about the present. It implies that the future is predictable.
It implies nothing about the future in itself. An index could gap up above the 20% decline level the next morning and no longer be in a bear market technically. In fact, the Dow may have only briefly dipped into bear market territory today. I haven’t checked, but I think it closed slightly above. Having said that, I believe the Dow has entered a secular bear, and may emerge (for more than a brief time) only after the easy credit/trashed dollar/balance of trade/current account deficit/transfer payments/real estate/inflation/deflation fiasco is cleaned up sometime after my kids graduate from high school (they are in elementary now). This doesn’t mean there can’t be echo bubbles along the way, even nominal new highs.
It is similar to the oft-quoted definition of recession. “Are in a recession” is only determined after 6 solid months of negative growth. You can’t know you are in a recession until a long time passes.
So “are in a bear market” is only determined after a 20% drop in the market. This is to distinguish the market formally from say a “10% correction in the market”. There is nothing to say the market couldn’t go straight into a 100-year long bull market the day after turning into a bear market.
But it does define the market. As in, “do you remember the bear market of 2008-2010?” It is just a label for a market that dips 20% off its highs.