These methods are not usually taught in economics courses. Why? Not enough data. In most cases economists are dealing with survey data, time series data and panel data. The benefits of cross validation and bootstrap/bagging grow with data size. When you're dealing with minimal amounts of messy/misbehaving data these methods lose their power.

Other methods become more important ie: Instrumental Variable estimation, Probit and Tobit models, Vector Autoregressions, Vector Error Correction models. Im sure your econ PhD student friend would know what these are.

A different tool-kit to solve different problems.