Won't argue with that, but it's a classic example of "Worse is better" [1]. It was simple and "good enough". Being ubiquitous is often more important than being efficient.
Most of the arguments for using HTTP reverse proxying over FastCGI or SCGI came down to ubiquity. It let you do things (like connect directly to your app servers with a web browser) that you couldn't do with FastCGI.