1) Hiring someone is a big deal. You want the people best placed to judge whether they're right involved.
2) You're selling the company to someone who may/will have other choices. Put the people forward who are going to do the best job of that.
Point 2 is important. I have no interest in candidates who do the whole "you'll make all the effort and bend over backwards because I'm in demand". Similarly I have no time for companies who demand days of preparation before interview as if I have no other options and they're the only game in town. In almost all cases the person isn't good enough to put up with that attitude and the company isn't the only place the person might consider working.
Jobs are a coming together of two interested parties and both sides need to show respect and be fair and reasonable. As a result if I'm recruiting then beyond a slightly tailored CV and some basic research into what we do (and maybe the technologies we use if you don't know them) I don't expect preparation. In exchange I'm going to spend time telling you about the role, the company and the project, I'm going to answer questions and I'm going to be as honest as I can be to make sure you can make a good decision.