Speaking as a former teacher of undergraduate professional and technical writing courses, in which one of the units was to create an employment portfolio including a resume, some of the reasons for teaching the students to create a 1-page resume are:
- to help students understand the very real situation in which they are one of many applicants, and the person reviewing the resume will spend very little time reviewing theirs
- to help students understand how to make rhetorical decisions about what to include and what not to include (because of the first point)
- to help students learn to write concise, action-oriented descriptive phrases about their skills and experience (also because of the first point)
In other words, ensuring students are not wasting a recruiter's time with fluff, at the point in time when fluff might be all they have.
But these guidelines for entry-level job seekers tie in to the general advice given in the Monster.com link used by another respondent, "The One-Page Resume vs. the Two-Page Resume", which focuses on understanding the short attention span of reviewers and showing the most recent and relevant accomplishments, among other things.
The guidance given to undergrads, that you and probably many others were given, is not materially different than the guidance we get as experienced, working adults, in which we
must understand the context in which we are applying (e.g. is it a cattle-call job in which first, quick, impressions matter most, or a more specialized advanced position in which greater detail is both desired and beneficial?) and tailor our resume to both the position itself and that context. This means making informed rhetorical choices which in the end may result in the use of a resume longer than 1-2 pages.
In some cases, that's just fine; I always suggest having two resumes in your back pocket: one made for a recruiter's easy scanning of skills and experience in the last 7-10 years (if you've been a working professional that long, and if the jobs in that timeframe are relevant to the job you're applying to now), and one to send to the recruiter and the hiring manager after you've made it to the "let's talk more" stage and giving a richer and more detailed picture of yourself is appropriate.