Michael Crichton has started his career with Andromeda Strain. A seemingly alien oriented novel.
In an interview he said that predicting the future is impossible and hence he did not give any insight into what he might think is the future.
Now is this primarily why he never gave any idea in his novel about alien MIGHT look? It is a queer move from a science fiction author to not have mentioned clearly in any of his novels. For example Clarke, H.G Wells and Stephen Baxter mentioned aliens in their respective novels.

Any other thoughts?