Nginx: Why is $host variable not available in stream directive in nginx.conf?

in my nginx.conf I would like to pass the request depending on the requested url to a specific server infrastructure.

Everything works great apart from the $host variable. I get error "nginx: [emerg] unknown "host" variable"

From my understanding is $host a regular variable and I dont have to declare it first.. in fact its used in the http directive above without problems:

http {
    ...
    fastcgi_cache_key "$scheme$request_method$host$request_uri";
    ...
}

But in stream it is a problem...

stream {
        map $ssl_server_name $targetBackendSSL {
            test1.example.com  192.168.1.1:22553;
            test2.example.com  192.168.1.2:22553;
        }
        
        map $host $targetBackendNonSSL {
            test1.example.com  192.168.1.9:22553;
            test2.example.com  192.168.1.10:22553;
        }
        
        # ssl
        server {
                listen 8000 ssl;
                listen [::]:8000 ssl;
                ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
                ssl_certificate /etc/letsencrypt/live/domain/fullchain.pem;
                ssl_certificate_key /etc/letsencrypt/live/domain/privkey.pem;
                proxy_connect_timeout 300s;
                proxy_pass $targetBackendSSL;
        }
        
        # non-ssl
        server {
                listen 8001;
                listen [::]:8001;  
                proxy_connect_timeout 300s;
                proxy_pass $targetBackendNonSSL;
        }
}

Any help appreciated! :-)