That's because there's no focus in the collection.
Theres a difference between collectors and hoarders.
People with 12+ pre order in their signature aren't most likely selective on the piece they want and either have a mansion/castle to expose them all (in which case they don't need storage advice and will probably never need to sell anything) or there's $3000 of figures cramped in a $80 non lighted non dustproof detolf to the point that you can only see 5% of some of the figures (basically the headscult and maybe the neck & shoulders)
They're most likely the ones who will sell 5+ figures in on go.
There's absolutely no problem with that tho, if that's how you like to collect.
The focused collector can have even items from the 80's or 50's he'll never let go because each piece is a matured decision with a special place and not the new spicy Friday PO that need shelf presence in itself due to poor shelf planning and non dynamic stand limiting the posing and inducing "boringness" in a figure.
If you do sell it there's plenty of alternatives for shipping, mainly foam you can cut yourself pretty easily, that is way better than the cheap plastics hot-toys 1/6 come with, but the real reason people keep the boxes is that they hope to get a profit from a figures and that's a whole other story.
e.g. vertical space. E.g. Origami 4-shelf large wheeled collapsible shelf - 
