While APs have a large amplitude, they last only very briefly and therefore don't contribute much to the EEG (machine isn't sensitive enough to notice such brief blips). EPSPs and IPSPs stick around long enough to be recorded.
Source: Garrell PC, et al. Introductory overview of research instruments for recording the electrical activity of neurons in the human brain. Rev. Sci. Instrum. 69, 4027 (1998); DOI:10.1063/1.1149245