Kafka doesn’t show the metamorphosis because he wants to deal with the aftermath of Gregor’s transformation rather than the actual process itself.
For Kafka, it’s not the actual metamorphosis that’s the main thing, but how Gregor and his family deal with it. That being the case, there’s little point in giving the reader a blow-by-blow account of Gregor’s transformation into a giant insect.
Not describing this process also leaves a lot to the reader’s imagination. We start to wonder exactly how the metamorphosis happened and why. There’s a sense of mystery about this bizarre and utterly grotesque transformation from human being to insect. As with any skilled writer, Kafka wants to leave us with something for our imaginations to work on without spelling everything out for us in bright primary colors.
If Kafka had described Gregor’s metamorphosis, then the story would have run the risk of becoming a horror story, which is not what “The Metamorphosis” is. Though Gregor’s post-transformation appearance is undoubtedly horrific, the subsequent events in the story are not really those we would expect to see in a conventional horror story, and so it’s perfectly understandable why Kafka didn’t feel the need to show us the eponymous metamorphosis.