Why is recruiting quality candidates considered as a challenge for hiring companies or Hiring Managers?

Lara,

Acquiring any resource (including Human resource) will always follow the Supply and demand.

In booming markets, where demand is high, you need to calibrate your offering to get the proper supply.

Calibrating your offering is not limites only to packages you are offering or approaches to retaining your your employees, but also your resources, tools, experiences, network, communities,...etc.

bare in mind that also good resources are well retained in their original companies so you need to package your self in a more attractive way.

Because you need to hire the perfect one for the companies to anyone and it's not easy to do this so HR is difficult job.