It's not actually. Most digital cameras pick up infrared light, especially cheaper ones. Maybe not professional cameras, which might have an IR filter for better color accuracy. The filters are not cheap and IR doesn't affect the image quality all that much.
Try it out; point your TV remote at your smartphone camera and push the buttons. You might be able to barely see the light with your eye in dark conditions, but the blinking pattern should be bright and easy to see through your phone.
I've tried this; you can definitely pick up the IR light with the front camera of your phone. Rear camera has an IR filter in modern cameras or something.
Every camera I've tried this with displays the IR as a pinkish purple color. You can sometimes see that same color in photos of things like sunlight reflecting off of chrome, or refracting through windows.
Try it out; point your TV remote at your smartphone camera and push the buttons. You might be able to barely see the light with your eye in dark conditions, but the blinking pattern should be bright and easy to see through your phone.