It's just that it's very easy for deviations from what the sensors see to occur. In traditional 3d printing, this can be stuff like a sensor switch wearing out, maybe physically moving slightly, being temperature sensitive, maybe the frame has changed shape slightly due to heat, moving the sensor a little, maybe something in the microcontroller happened to cause a slight delay in reading the sensor, or looseness developing in the motion system, or something being slightly out of alignment, or some component in the extrusion system experiencing momentarily higher friction and so on.

The layers are really thin, so manually inspecting them would slow down the printing process drastically. Then, ultimately, what even can you do if there's a defect? The layer has been laid already. If material is missing somewhere, you could have the machine go back and add it, but if there's excess material somewhere, or it's in a form that the machine can't fix, there's not a lot to be done, particularly in applications like rocketry, where your structural strength tolerance are very tight.