system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do art museums only ever display a tiny fraction of the items they own?

As I understand it, a typical big-city museum has at most 5 or 10% of it's total collection on display at any given time. Some items will be rotated in and out but lots are permanent fixtures, meaning many (if not most) of the items in their possession will never be seen by even a single museum visitor, including works by acknowledged masters.

Couldn't a museum sell a substantial number of items to other museums (or to private collectors) and reap a substantial windfall? And wouldn't smaller museums benefit tremendously from acquiring lesser-known undisplayed works by major artists that are currently gathering dust in the basements of the biggest museums?
gpt
1) Light can be damaging to many things. This can be limited with special displays but it is expensive.

2) Air can be damaging to many things. Like the light issues special displays can be used but they are expensive. 

3) Space. Most museums do not have the space to display everything. 

4) Quality. Not everything is of display quality, but is still of historic or scientific importance. 

And museums do sell/share with each other a lot. But how, when, and to whom a museum is allowed to sell things is fairly highly regulated. They are not generally allowed to sell to private citizens.