Why do eggs lack an endoplasmic reticulum?
I could find no indication that eggs are lacking an endoplasmic reticulum. On the contrary, I could find papers written on the ER in eggs of several species. 

Mouse: The onset of activation responsiveness during maturation coincides with the formation of the cortical endoplasmic reticulum in oocytes of Xenopus laevis

Sea urchin: Structural changes of the endoplasmic reticulum of sea urchin eggs during fertilization

Human: The relationship between pregnancy outcome and smooth endoplasmic reticulum clusters in MII human oocytes

Frogs: The onset of activation responsiveness during maturation coincides with the formation of the cortical endoplasmic reticulum in oocytes of Xenopus laevis

However, as can be read in the sea urchin paper, the ER changes after fertilization:


  We interpret these observations to indicate that in the first few minutes after fertilization, the egg's ER is fragmented


For other species, I could only find evidence that the ER reorganizes after fertilization because of changed calcium signaling needs