Why pythons look like boas and boas like pythons.

Black headed python - Wikipedia

It's called convergent evolution.
Different species look alike even though they are not related. Hares and rabbits look alike but they are not the same species. Boas and pythons are another example. They don't even belong to the same family. Yet some of them look just like each other - though their common ancestor was 70 million years ago.
An aquatic python has a head like an aquatic boa and a land dwelling boa has a head like a land dwelling python.  Read more here
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