Why won’t the Obama administration reveal how many Americans’ emails the NSA has collected and reviewed without a warrant?

"Since last year, a few members of Congress—led by Senator Ron Wyden—have been trying to get the Obama administration to answer a simple question: how many Americans’ phone calls or emails have been and are being collected and read without a warrant under the authority of the FISA Amendments Act of 2008 (FAA)? Unfortunately, no one else in the government seems to want that question answered."