It's not even so much the fact that they're old (although that's a factor); it's more that you've just sort of raced through about a decade that should've been crucial to their relationship. In that decade, Jack grew up, Wilson got married to some broad, and House... must've been doing something, but we don't know what he's been up to. The years fly by, nothing is said about how Wilson and House are coping with this, how Wilson managed to stay faithful to the broad (or did he), whether anything at all happened to either of them professionally (are they still working side by side and eating lunch off each other's plates?)... Whether Wilson has been at all happy. Whether House has shrugged it off quickly or suffered. Whether they've been together at all during this time. Leo saw thim kissing during Jack's graduation - is that even normal for them anymore? One gets rather the impression that after House's decision to bolt, they both brutally killed their romance altogether, House out of a perverse sense of sacrifice and Wilson out of aan inability to go on pining for what House wouldn't let him have. And now that they're both quite old, and their passion is eleven years in the past, it's kind of hard to imagine they'd even want to get back together. At this point, they're probably each resigned to a sort of low-key misery. Even if you got them back together now, what the heck is the point? They've wasted their allotted time.