There’s two issues here that are likely contributing to your troubles. First, you can’t check for file existence so easily in a server context (as you are with httpd.conf), because the request hasn’t been mapped to the file system yet.
If your requests for static resources happen to correspond directly to your file system structure beyond your DOCUMENT_ROOT, you can solve this by appending the value of DOCUMENT_ROOT before the incoming REQUEST_URI, as follows:
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} !-f
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} !-d
If the path to your static resources is transformed somewhere down the line, the mod_rewrite documentation suggests using the following, which will issue a subrequest to perform a lookahead based on the request URI:
RewriteCond %{LA-U:REQUEST_FILENAME} !-f
RewriteCond %{LA-U:REQUEST_FILENAME} !-d
This approach is more expensive though, so try to use the first one where at all possible.
Secondly, the URI that you pass back into the request is actually invalid, because it needs the leading slash. Correcting that and combining it with the new conditions above gives you this modified rule set:
RewriteEngine On
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} !-f
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} !-d
RewriteRule ^.*$ /index.php?page=$0 [PT,QSA,L]
Note that the value of page here will always have a leading slash, so you might actually want to use this rule instead, depending on your handling logic in index.php:
RewriteRule ^/(.*)$ /index.php?page=$1 [PT,QSA,L]
