PHP isn't either, except that it's packaged well in Linux.
If there were good packages for Ruby in Debian, the only advantage that PHP would have is in being the shared hosting default. Both are pretty equally performant, and Ruby has the advantage of being a nicely designed language.
>there is absolutely nothing to be gained by not using PHP.
What is there to be gained by using PHP?
<?= 'Hello World' ?>For example in railsland, typo and mephisto were both popular at some point, and in djangoland there have been at least two dozens blog engines _I know of_.
There was a beautiful django conference video where the speaker asked "who is writing a blog engine in django" and about 9/10 of the room raised their hands.
EDIT: I concur, many language advocates believe that, but I think they are wrong for different reasons.