it does check continuously. i'm guessing the intervals aren't narrow enough for you. perhaps you're a hypochondriac. who knows.
healthwise, your heart rate is not supposed to widely fluctuate under normal circumstances. it changes based on oxygen demand so it changes the most during exercise, a disease state, or severe stress/duress. why would you need it to check every millisecond when you're just sitting there? under those conditions, your heart rate should be stable.
I found the heart rate meter to be continuously checking my heart rate. Maybe it's not every millisecond but it's good enough during rest. Ad when you do exercise, it narrows the interval. So what's the issue? The intervals are narrow enough for exercise and narrow enough for rest.
the true usefulness of these monitors isn't the absolute number, it's the trend the number indicates. if the person are truly under duress, you'll see it all over the person. it won't take a monitor to tell you the person is in trouble. he'll be sweating, have a feeling of heaviness, cloudy mind, etc. whether your monitor says your baseline is 70 and your HR increases to 200 or whether your monitor says your baseline is 80 and increases to 210 says the exact same information. It's the trending that's most important.
And when you go to a doctor. He doesn't look at a single number. He looks at a trend. If you're doing a stress test, he's looking at the plasticity of your heart and how well it recovers after the exercise is completed. If you're just doing a normal checkup, he's taking your value and seeing whether you're progressing or regressing, healthwise, against your chart history. He's not going to measure your heart rate for 20 minutes every 30 seconds.
I've seen patients demanding antibiotics. Demanding a CT scan. Demanding a mammogram. All when their cases don't warrant it. Testing more often doesn't fix the problem. A CT scan has 600x the radiation of an X-ray. You shouldn't be testing on a whim. If your doctor is telling you to get these scans all the time, you should check his credentials. A good doctor wouldn't overprescribe broad spectrum antibiotics either. A good doctor would use a narrow spectrum antibiotic if the situation demands it or start off with a broad spectrum and dial down as the offending organism is deciphered. Mammograms haven't show to decrease death rates at all. Isn't that the point of testing? To decrease death rates, increase health indices, etc.? The point of testing isn't to just test.