That’s because nobody has been able to count the total number of federal laws on the books.
A post at the Library of Congress’s blog put it this way:
At the reference desk, we are frequently asked to estimate the number of federal laws in force. However, trying to tally this number is nearly impossible.
If you think the answer to this question can be found in the volumes of the Statutes at Large, you are partially correct. The Statutes at Large is a compendium that includes all the federal laws passed by the U.S. Congress. However, a total count of laws passed does not account for the fact that some laws are completely new; some are passed to amend existing laws; and others completely repeal old laws. Moreover, this set does not include any case law or regulatory provisions that have the force of law.“]
You can read the whole post here.