The primary use-case for is ephemeral content -- code snippets which will be shared shortly after posting, and maybe never seen again. Other uses merit longer-lived items, but it is not intended to be a publishing service.

A major reason not to have a "never expire" option is spam. You wouldn't know it, but the site is very actively used by spammers. Some spam-bots are smart enough to choose the maxiumum expiry. I have anti-spam automation that detects most spam, but even spam misssed by that process expires eventually. If "never expire" was an option, spam would be even more of a resource drain and maintenance burden than it already is.

Also, some users running automation create thousands of items in a day (without even violating the Terms of Service). They often choose the maximum avaliable expiry (365). If there were a "never expire" option, many would surely choose it, and the database would grow without limit.