The answer is that public recordation of a patent assignment is not required for the assignment to be a valid transfer of ownership. See, e.g., this discussion in the Manual for Patent Examination Procedure:
Recordation of the assignment provides legal notice to the public of the assignment. It should be noted that recording of the assignment is merely a ministerial act; it is not an Office determination of the validity of the assignment document or the effect of the assignment document on the ownership of the patent property.
Why then are assignments recorded at all?
The answer is partly that under Section 261 of the Patent Act, an earlier assignment recorded within three months from its date of execution (or otherwise before any later assignment) voids any later competing assignment of the same patents to a buyer who had no actual knowledge of the earlier assignment, even when the later buyer paid a lot of money:
An interest that constitutes an assignment, grant or conveyance shall be void as against any subsequent purchaser or mortgagee for a valuable consideration, without notice, unless it is recorded in the Patent and Trademark Office within three months from its date or prior to the date of such subsequent purchase or mortgage.
Patents in this sense are treated like real property. The recording act for patents is an example of a "race-notice with grace" rule of priority, and each state has its own slightly different rule of priority in recordation. Patents, being a creation of federal law, are governed by a federal statute.
The other part of the answer is that some patent owners prefer to keep confidential the full extent of their patent ownership for strategic reasons. If a transferee of the patent rights isn't worried about the prior owner fraudulently trying to sell the same patents to another buyer, then she may wait or avoid recording the assignment to conceal her ownership (and whatever strategic information that might convey) from the public. Partly as a consequence of this, gaining freedom to operate can be quite difficult for a business seeking to clear patent rights!