J Mascis has nothing better to do
The most interesting detail here is that Mascis doesn't just give short answers. He gives deft nonanswers, which suggests real skill in deflecting without being rude. That's a specific craft, and it says something about how much thought goes into maintaining a supposedly effortless image.
I think there's a real tension between the slacker aesthetic and the precision of his guitar work. You don't reach that level of playing without obsessive practice, so the onstage stillness and the interview evasions read as a deliberate refusal to perform the role of the eager artist for the press. He's letting the music do the talking, and that's a fair position.
The question I'd want answered is whether this is consistent across years or something he's refined. If every interview is the same wall of polite deflection, that's a persona. If it varies, it might be genuine shyness. Either way, it's working, because we're still talking about him.