I published the first systematic study of the escape of ionizing photons from resolved stars in GMCs into the intercloud gas via a large set of radiation-MHD simulations (He, Ricotti & Geen 2020). We found that $f_{\rm esc}$ increases with decreasing mass and with increasing mean density of a GMC. GMCs with densities typical of local star formation regions have negligible $f_{\rm esc}$ (below 10%). This relation is explained by a simple model where two timescales are compared: the lifetime of the dominating UV sources (O/B stars) and the cloud destruction time. The former is nearly constant for massive clusters and slightly higher for less massive ones where O/B stars are lacking. The latter is found to be several cloud crossing times of the HII fronts at ~7 km/s, which is the characteristic timescale of photoionization feedback. We concluded that the sources of ionizing photons responsible for the epoch of reionization, one of the most important yet poorly understood stages in cosmic evolution, must have been very compact star clusters forming in dense environments different from today's galaxies.