I didn't realise that. It's a bit confusing because of some elements (Cast/Downloads) not appearing when not available but Captions do.

+amyroberts@ to confirm it's on purpose. (I would be interested to hear the rationale in general so I can better understand :))

It's something we debated. More on the background here: https://docs.google.com/presentation/d/1wso086QMuxxSyIJPuuv2T5aJOn3Mlwo4c9KhfAdMdGw/edit#slide=id.g2353fa4db1_0_0

Ultimately we decided to err on the side of predictability and Material guidance:

"Menu items that are irrelevant to the current context may be removed
Menu items which are relevant but need certain conditions to be met may be disabled. For example, the Copy menu option becomes enabled when text is selected."

If a person doesn't have a cast device, there's no context in which they'd become relevant so we decided that's the item that would be hidden if a cast device wasn't detected. 

CC and download though are variable based on conditions set by the developer. In order to help users understand where to find these elements across Chrome predictably (not just on a particular video), we decided keep them visible but disabled. 

Oh so download should also appear and be disabled when not available?

I believe so!