Old 401(k)s are a little like the old clothes in the back of your closet. You know you should do something about them--try them on and see if they still fit (and aren't hopelessly out of style), or donate them to charity. But inertia isn't costing you anything other than closet space, and you know the process of getting everything in shipshape could take a little time. So, there they sit--mostly out of sight and mind.
And so it is with your old 401(k). Thanks to a strong stock market, the periodic statements you receive show that your account balance has probably ticked steadily upward since you left. And while you've heard that you should consider a rollover, you've also heard ominous tales about how you can end up on the hook for a bunch of taxes if you don't follow the rollover rules to the letter. The path of least resistance beckons.