Why should he? If it gets too bad, he'll fix it like so many other celebs. Only hateful people like the OP care and use it to mock Darren. Speaks a whole lot about them, nothing about him.
Coming back to where you started is not the same as never leaving. ~ TP

I've noticed ever since A Very Potter Sequel. But he doesn't seem to mind.

You do realize it's likely in large part the lighting combined with that specific angle, right?
I mean he has a little thinning area toward the back of his natural part, yes, but that video emphasizes it a whole lot because it's shining bright light directly down into the area of his hair that's a little thinner. Any other angle and less harsh lighting, and it doesn't look nearly that bad. There's still hair there, and it's big and curly in texture so it does a good job in general of not seeming that terribly thin.
And I freaking love his hair with its big curls, so hell no I would never advocate for him shaving it! haha. I mean at some point he may well start legitimately going bald (I sure hope all the gel they pile in his hair for Glee isn't speeding that up!), but for right now, it's just fine.
This video is a much more accurate view of how it really looks:
You can see his scalp a little bit through the hair toward the back, but nothing like that other video.
Oh Bambi, I cried so hard when those hunters shot your mommy. - Kurt Hummel