This would only work on connectionless protocols, such as UDP. UDP offers no message reliability and relies on the source address to respond. This is how DNS amplification attacks work, as the main protocol of DNS is UDP.

Whereas HTTP goes over TCP which is a connection-oriented protocol. TCP offers message integrity by going back and forth between client and server multiple times to verify the message was retrieved successfully. Without a valid source address the 3-Way TCP Handshake used to establish the connection cannot succeed.

But the only point in this is to take down a site you won't be able to get into useful information back from this request as the request's response will be heading back to the spoofed address... (though if you are using for ddos it is pointless to get the data back...)