It's clear that dashes (-) are better than underscores (_) for search engine optimization. So people have looked for ways to make MediaWiki use -. Wikihow is one of the few MediaWiki sites that behaves this way, but unfortunately even they don't seem to be running an extension for this.
I tried my luck at changing the MediaWiki core (Title.php) but I'll leave this for now.
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See original: Underscores to dashes in MediaWiki (not yet)