Companies posting job ad does not necessarily equate to companies hiring. Some of these ads may be disingenious - they still interview candidates only to have a list of desirable people to make offers to once the company starts hiring again.

And of course, on top of that there's standard recrutiment scam of "oh, sorry, this position was just filled, but may I interest you with $WORSE_JOB_FOR_LESS_MONEY instead"?