A lot of prominent plugins require the ability to modify .htaccess. With Nginx hosts, the users have to do it via a web panel by adding the rules manually. No non-technical user can do that. So the majority of WP space still runs behind Apache.
Its not. Literally almost ~50% of the web runs on it. Apache has top level permissions. Then individual virtual hosts/sites can modify their own htaccess per the permissions that they are afforded. These permissions are generally limited, but when it comes to mod rewrite, they have pretty much full liberty to write any rewrite rule. Since their rewrites would affect just their own vhosts anyways.
Works like a clock. And its extremely convenient even for developers. Anything ranging from seo friendly urls to anti-bot/security rules can be written in by a plugin.