<p>The simple answer is that LACs are colleges that don’t have any graduate programs, and generally don’t offer the pre-professional majors (although there are exceptions), so the faculty are focused on undergraduate education, rather than research and there are no graduate students to be TAs.</p>
<p>Most top LACs have excellent math and science programs, although they won’t offer graduate coursework in those areas. Harvey Mudd and Sarah Lawrence are actually exceptions to the general LAC rule. The top LACs generally are strong in the sciences, the humanities and the social sciences, although how strong in a specific dept. will vary by school. Because the schools are smaller than universities (generally student bodies are under 2,500 students), there are fewer majors and, as I mentioned, only rarely pre-professional majors (engineering, accounting, nursing, architecture, journalism.) </p>
<p>There are lots of threads on the advantages and disadvantages of LACs vs. universities so I won’t repeat those arguments here. Just do the search.</p>