Often they've been lead there under false pretenses, such as being told that compensation is as much as double what is actually your realistic cap. Or that they will be in a far more senior position than you are interviewing for. In that person's eyes it is you, the interviewer, not they the interviewee, who is in the begging position.
So you are effectively reaching out to candidates, pre-selecting them and asking them to come interview with you. To the candidate, they are doing you the favor, not the other way around. If they show up and you are not appreciative that they took the time out of their schedules to meet with you, they are going to be less than impressed. They assume that you have sifted through large numbers of candidates and selected them for a reason.
Using a staffing firm is never advised, in my opinion. They do not represent the interests of you as the hiring company nor of the candidates. They, at best, are a point of miscommunications and increased cost. At worst they play both parties against each other for their own gain.
Like an HR department, they have very little to add to a selection process but have the capacity to wreak nearly unlimited damage. The best companies, no matter what size they are, take the time to make their hiring process a purely internal one.
The bottom line is that your hiring practices dictate what you are and will become as a company. If you don't acquire and develop good staff you won't have them to drive efficiency and innovation. Take your hiring process very seriously and consider how your company presents itself to a candidate.
Remember, weeding out bad people is easy. Attracting good ones is hard.