And there are several reasons that I, at least, do not offer a single zip of all my packs. Space is the biggest issue.
I have over 8GB of packs. That alone is a significant portion of the available space on my Mega. I use a free account, so I am quite limited. Hosting a combo zip would double that, leaving me very little space available.
Plus I would need to change out the combo each and every time I add a new pack, which would mean 8+ GB of additional upload bandwidth to get it onto the Mega.
Plus very few people would want everythingfrom 300+ packs. Individual pack downloads let people pick and choose which packs they actually want.