Why do companies are hiring consultancies???????????

I have a doubt in my mind that why do most of the companies are hiring consultancies to hire the candidates……
I mean to say, can’t they hire best candidates or they don’t have such time for this process….
Even hiring candidates itself is much cheeper than through the consultancies…
and also for candidates that they don’t get the right package…
will anybody tell any reson for that??
I will be highly thankful to all of you for this.