I don't understand the reasoning behind it being okay for breastfed babies to go several days or…

I don't understand the reasoning behind it being okay for breastfed babies to go several days or even a couple of weeks without pooping. My son hasn't pooped in a week now but he grunts and tries to push. Previously he went three or four days without pooping and when he finally did it was a blowout! Supposedly the babies use everything in the breastmilk so they don't have any solids to expel, but if that's the case, why do they usually have blowouts when they finally go? 🤔😑😕