Because it's really hard to build, test and maintain a non-trivial application in a dynamically-typed language like JavaScript.
I think it's telling that a lot of companies with the most mature products (Facebook, Dropbox, Asana) eventually resort to optional static typing in their products, whether that means extending their runtimes (e.g. Hack) or using a transpiler like TypeScript.
The implicit conversions and "we'll just randomly give you undefined to propagate ondwards" stuff is a much bigger source of bugs in JS than dynamic typing. Picky dynamically typed languages like Erlang or Python will pretty reliably produce an error/exception at the scene of the crime. (More so than C/C++!)
(Also I don't think optional static typing has gotten off the ground in Python land, or adopted in production at Dropbox?)
What is it telling? It might just be telling that everyone is doing what you're doing here, and looking at each other to see what they should do. Another name for this is "fashion."
I think it's telling that a lot of companies with the most mature products (Facebook, Dropbox, Asana) eventually resort to optional static typing in their products, whether that means extending their runtimes (e.g. Hack) or using a transpiler like TypeScript.