It's just not that exciting or important to them. So, they know they should put in more effort to be romantic because it's something their woman wants, but that's usually the only reason. Otherwise, it's more of an issue of guys giving slightly more than they need, which isn't much at all for most men. As a matter fact, men are much more emotional than most would believe. But it's different than it is for women.
Men get emotional over different things, like sports and driving.
They do not lack emotion. These men aren't broken, that's just the way they are. First, it changes what things mean such as his lack of enthusiasm for things she cares about, i.e. romance, intimate sex, time with her, etc. etc. It's not that he's bored with her, it's that he is boring. His flatline state is intact and yet she feels something is missing.