dict.get() is a method call wheras "key in dict" and "dict[key]" are operators. Method calls are still slower than operators, even after past optimizations. For example, when dict.get was converted to METH_FASTCALL, it was around 20 ns faster:
https://vstinner.github.io/fastcall-microbenchmarks.html

See also closed bpo-17170 "string method lookup is too slow".