Why does the spaceship operator have only one equal sign in it?
Why would it have two? There's only one in <=, >= and !=. It's not inconsistent at all. Only == is inconsistent, and that's to avoid conflicts with the assignment operator.
The spaceship operator is a combination of a < b, a == b, and a > b. Presumably, the single equals sign was chosen for the same reason it was chosen for >= and <= -- it's simply shorter and easier to read.