Time lapse videos are created from photos taken over a period of time. If you create a 30fps 10 minute video, that's already 18000 photos.

It's quite a pain in the ass to import, color grade, deflicker etc. So one would usually wait till there are a few projects are taken, and process them all at once. If this is your hobby, you can easily hit 5000000 photos in a couple of years. Especially if you're doing something like an ultra long time lapse of say, the construction of a building using two cameras.

If the question is "why don't you delete the photos after creating the time lapse video?" It's called photo hoarding haha.

Anyway that's irrelevant, because if I paid for a certain storage size to backup my hoarding habits, I expect to be able to use the capacity, and not have some random ass limits.