Excellent point, have thought the same thing? This is usually when the conspiracy theorists chime in with "tom would only do it if he sang more" argument but I don't think that's it
When I listen to Neighborhoods, I get the sense that Tom and Mark wrote separate from each other. It's like Tom went "hey, here's a verse" and Mark came up with a chorus. There's no real back and forth between them, either. Shit, I half believe that "Heart's All Gone" is a shot at Tom. But that's just me.