> By Ruby programmers.
As well as by not-Ruby programmers, which is why they keep getting used in non-Ruby languages that don't keep other aspects of Ruby.
> Why is it that Ruby programmers are incapable of imagining anyone doesn't love Ruby?
I think most Ruby programmers are capable of understanding that lots of people don't like Ruby, and more importantly that lots of people don't like aspects of Ruby that those Ruby programmers find most attractive.
Sure, some Ruby programmers have problems with this, but, OTOH, some non-Ruby programmers seem incapable of understanding that other non-Ruby programmers might find some aspects of Ruby attractive that they, themselves, do not.
> I've met plenty Java programmers willing to admit the shortcomings of Java
Well, sure, the more likely you are to be compelled to use a language because of corporate inertia, the more likely people that actually do use it will be the people that don't like it and use it under duress. So I'd expect Java to be among the languages with the highest proportion of active programmers who not only are willing to admit its shortcoming, but who outright despise it.
> I've yet to meet a Ruby programmer who doesn't believe that Ruby is the best thing since sliced Jesus.
Given the number of non-Ruby programming languages created with substantial input from (or even under the leadership of) people in the Ruby community specifically because, while they like many things about Ruby, they find it not optimal for some uses, I think this says a lot more about your limited exposure to the Ruby community than about the Ruby community itself.