Just to clarify: Many LACs do offer some grad programs (the CDE and Art History programs at Williams, for example), but they generally do not offer undergrad majors in “pre-professional” disciplines or sub-specialties. In other words, at most LACs you won’t be able to major in “Poetry” but you’ll be able to major in “English Literature,” you won’t be able to major in “chemical engineering” but you’ll be able to major in “chemistry” (although the STEM LACs like Mudd do offer engineering, as does Swarthmore, somewhat uniquely).
LACs are typically very small and exclusively (or almost exclusively) undergrad. They support and exemplify the notion that undergrad is still an important time for general/broad education and that graduate school is the appropriate place for further specialization or career preparation.