The answer is very simple.
The companies should focus on core business activity and leave the hiring to consultant. The reason is you deviate from core business activity by running a huge HR department handling the hiring. I can quote an example, To conduct the marriage for son/daughter we should not buy the marriage hall.
Secondly HR consulting firms are specialised in sourcing the right talent using various strategies and does add value in sourcing.
Thirdly HR consulting firm is the face of the client and boosts the image of the company by marketing the client to candidates
Lastly HR firm does end to end job which makes the client job simple and they do not invest time