Partly. But I suspect it's more that CGI was the way things had always been done. They didn't even consider doing a reverse proxy. They asked the question "how

Partly. But I suspect it's more that CGI was the way things had always been done. They didn't even consider doing a reverse proxy. They asked the question "how do we make CGI faster" and so ended up with FastCGI. Other developers asked the same question and ended up making mod_php (and friends), embedding the scripting language directly into the web server.