That might not always be true. Some people have saved up for a long time to get their first pistol or shotgun, and they might be a long way from buying that gun safe. Other people I know prefer to leave their firearm on their bedstand or hanging above the mantle.
Along those lines: if you have an actual gun safe, it’s probably a good place for storage. It’s lined and meant to hold firearms. If you had to settle for something cheaper, I would just make sure your firearm isn’t making metal-on-metal contact. I’m not a gunsmith, but I used to be a repairman, and this was a big deal for us. Wooden toolboxes were the gold standard; wood will naturally regulate humidity and keep steel in good condition. You wouldn’t buy a wooden safe, but you can line your safe with natural materials that help keep it nice inside for your “tools.” I’m making an assumption there, so I’m willing to be corrected if this isn’t true for firearms.
Also, keep in mind that storage may not be as big a deal if your house is heated in the winter and air conditioned in the summer. Your firearms are probably already treated, unless you have some sort of antique revolver that’s just raw steel. So unless they’re just lying on a metal shelf in the shed, they’re probably doing pretty well no matter how you store them.
The only issues I’ve personally encountered come from the wrong kinds of oil that gum up the mechanics when they aren’t used for an extended period of time. Looks like we have some good opinions on that, already. I’d be curious to hear from any gunsmiths on this board, what kinds of issues they’ve seen from improper storage.