It's far cheaper to use a storage unit for a lot of this stuff I do routinely use than buying and selling a house just to get more storage space at home.
I'm not blind to the fact not everyone just stores junk they never use. I'm certain there's a percentage of stuff in mine that is probably better off sold or trashed. But until we stop changing decorations for the different seasons, stop a number of hobbies with bulky stuff, or move to a giant mansion I just don't have the space and could use a few more square feet of pure storage space.