I think employment agencies say this because they are in the habit of retyping and reformatting resumes and longer resumes take more time to manage.
I started to have better responses when I started ignoring the advice to keep the resume to one page.
The number of pages is also ambiguous when everything is transmitted electronically, I always provide a link to the on-line HTML version, and sometimes attach a PDF generated from that. It's not actually that long when displayed on-screen.
I have personally not had any trouble getting interviews because I have a 3 or 4 page resume (once printed out). In order for it to be one page, I'd have to exclude a significant amount of my work experience, work experience which is relevant to the jobs I'm applying for. The resume should be an indication of exactly what you can do, your experience and capabilities should be laid out before the interview. The interviewer should be able to look at your resume and say "This person has the skills and the experience we need, let's talk and find out if they're a good fit", not "I can't tell what this person is capable of because their resume is not very detailed".
Also, I try to write my resume with a focus on technical readers, not necessarily HR. Part of the goal is to get past the resume screeners, but if you want the job, having something to talk about, prompted by the contents of the resume, during the technical interview with actual technical people is a big advantage. The last thing you want is to have a sparse resume such that the interviewer does not have a basis for asking questions.
I guess this can kind of come off like "pre-loading the interviewing process", but when I've had to be the interviewer, the better interviews (and the better candidates) were the ones who already knew what they knew and didn't need to spend time reviewing their own past during the phone or face-to-face interview.
Your resume is an invitation to interview. It should be used like an advert, short and to the point and highlighting your best features.
My 1-page resume always impresses people much more than my 2-page resume.