Because it's a relatively new uni, so not as prestigious to people who have no idea what they are talking about. Warwick is one of the best unis in UK for Econ and Math, right after Cambridge and LSE I'd say for undergrad econ along with UCL, and is also very good for Engineering and Law etc and the employers know that. Not 'overrated' at all. Though I'm not sure the location/social scene is the best though, but hey I've never visited.