Why Does “=” Mean Assignment?

It does! Even though, Martin Richards' homepage [0] hasn't been updated in a while you can try it out with a small amount of fuss [1].

0: http://www.cl.cam.ac.uk/~mr10/

1: https://gist.github.com/seaneshbaugh/e09abd748ccc07c5463f253....

Another interesting way to think about it is as "match". That is try to match the stuff on the right with the stuff on the left.

Take Erlang for example:

    1> X = 1.
    1
    2> X = 2.
    ** exception error: no match of right hand side value 2

Notice variables are immutable (not just values themselves). Once X becomes 1, it can only match with 1 after that. You might think this is silly or annoying, why not just allow reassignment and instead have to sprinkle X1, X2 everywhere. But it turns out is can be nice because it makes state updates very explicit. In complicated applications that helps understand what is happening. And it behaves like you'd expect in math, in the sense that X = X + 1 doesn't make sense here either:

    1> X = 1.
    1
    2> X = X + 1.
    ** exception error: no match of right hand side value 2
    3>    

It does pattern matching very well too, that is, it matches based on the shape of data:

    1> {X,Y} = {1,2}.
    {1,2}
    2> X.
    1
    3> Y.
    2
    4>

In other languages we might say we have assignment and destructuring but here it is rather simple it's just pattern matching.

The length of symbols should roughly be negatively correlated with its frequency.

Since in procedural language, assignment is a much more common operation than equality check, it is reasonable to favor "=" over ":=" or even "<-".

Traditionally? I don't think there was a "tradition". ":=" was used in Algol. "=" was used in Fortran. COBOL didn't use either "=" or ":=".

Yes, and Martin Richards still maintains it himself. He even uses it for his music processing research.