It's kind of hard to agree with your statement albeit it has some merits.

Rails is a glue of a bunch of things. It's not like it doesn't exist in other environment.

Meanwhile, the state of JS testing is still far behind. Like really really far behind.

Once you get that sorted out, you still left with common set of libraries. Still a long way to go really.

Not to mention that the whole client-side thing is ripe for a change (depending on whom you talk to).