Incorrect, you can vote on closed questions. You just can't add new answers.
Anyhow, SO isn't the same site it was 5 years ago. It's actually pretty darn toxic for new userbase and seems to cater to the mods themselves.
That's another frustrating gap in SO: the absolute ignorance of the notion of time and change. Both questions and answers often have a significant timeliness attribute (ex: what are the best online resources for learning <x> as of Jan 2015), and SO has no support for this. To be fair it's a hard problem, but I'm not even aware if it has even been discussed.
I'd like to think that the platform is now mature and tuned enough that they could start to introduce some major new functionality rather than just keeping the lights on.