At the end of the day it is really about how much existing code can be quickly reused.I used to think this was true, and then after I became a bit experienced with web development, I realized that the bulk of code (JavaScript, in particular) released for public use ("reusable") is garbage. Complete, utter garbage. Poor APIs, terrible abstractions, namespace contamination, a lot of assumptions that those packages are the only ones in use.
Yes, you can slap something together quickly with the bits of turds available online, but when it comes to customization and fine-tuning, you will either spend a lot of time undoing half of what some other developer did with their package or writing one of your own. For writing my own widgets, tools, and frameworks, I want a great language. And when I build tools with a great language AND with a good language design approach (it takes both), I have things I can reuse.
More available reusable code is not an advantage. I am coming to think that the more accessible a language is for just anyone to use, we get lesser quality shared code.
There are gems, for sure -- I will happily name jQuery as an excellent, extensible framework. A lot of the community's jQuery plugins, on the other hand...