Because the virus spreads at an exponential rate, cutting off the spread immediately (as Dr. Fauci demands) is what the nation is doing.
But, to get a more accurate picture of how much it has spread to date, we need to freeze in place, and wait 7-10 days. The # will be higher than you think.
Yet, the # will be significantly lower than what it would otherwise be, assuming we did not take drastic measures.