It is the way the .htaccess wordpress generatea instructs apache to work on the wordpress directories
The relevant parts from the default .htaccess
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
The first line will cause the rewrite engine to bail out if there is a file with the same name as the server is asked to serve
The second line will cause the rewrite engine to bail out if there is a directory with the same name as the server is asked to serve
The third line is what actually invokes wordpress and you will not get to it if you bailed out on one of the first two lines.