Long URLs constantly disappear, but only one root at a time. If Wikipedia disappeared tomorrow, all the Wikipedia links would break, but URL shorteners consolidate roots. If bit.ly disappeared tomorrow, it's not just one site that is affected. Furthermore, the use of a redirect breaks traditional mirroring systems. You can't just prepend somemirror.com to the path and expect it to work.
Although I'm not sure why people are so terrified of short links breaking; long URLs are constantly disappearing.