Bruce Schneier explains why making the haystack bigger doesn’t help us find a needle that keeps changing:

For example, even highly accurate medical tests are useless as diagnostic tools if the incidence of the disease is rare in the general population. Terrorist attacks are also rare, any "test" is going to result in an endless stream of false alarms.

Data Mining for Terrorists, Bruce Schneier, March 09, 2006, Schneier on Security.

And many false alarms mean a lot of innocent people being harassed needlessly, in other words, civil liberties being eroded for little or no benefit.