Everybody wants their pages to be easy for the world to find, but getting your site listed can be tough. Search sites may take a long time to list your site, may never list it at all, and may drop it after a few months for no reason. If you resubmit often, as it is very tempting to do, you may even be branded a spamdexer and barred from a search site.
Spammers are to blame for a lot of the search sites' problems.
This means that they have less time to devote to keeping their databases current and their search algorithms tuned. It also means that a lot of perfectly worthwhile sites get barred from, or never added to, many search sites. See Keeping the Search Engines Happy for more ruminations on the spam wars, and how to avoid being caught in the middle. Spammers aside, many of the problems stem from the simple fact that the search engines are swamped. Like most everything to do with the Internet, runaway growth has resulted in massive backlogs.