Sure, they're easy to see, but it doesn't immediately follow that they're easy to hit or otherwise do something about. Furthermore, the shuttle is a lot bigger than ICBMs are, and the shuttle, having humans aboard, accelerates a lot more slowly as well. I've seen a video of an ICBM launch, and that thing really moved! It was from here to GONE in a few seconds. Not necessarily easy to hit, even if you can see it.
For 14 years, I worked with lidars that would track the brightness of a pulse of light as it traveled away from the laser. Even at a slow sampling frequency of 40 MHz, a pulse of light could be tracked every 7.5 meters.
Compared to a pulse of light traveling at 186,000 miles per second, and ICBM is almost standing still when tracked with today's electronics.