Why "exclamation mark" but not "exclamation sign"?

Solution 1:

I suppose it's in the origins.

? and ! are from the family of language punctuation marks

= is from the family of mathematical symbols or signs.

Solution 2:

I think they're called marks because they actually mark something. That a sentence is a question, an exclamation, or a quote.

But = is just a sign, it doesn't mark anything.

Again, this is just another guess.

Solution 3:

Another difference I see is that the exclamation mark has no meaning on its own. It is merely punctuation whereas the equals sign actually adds meaning to the sentence: life "equals" goodness.

Solution 4:

I cannot comment yet, as my karma will not allow it. The exclamation mark was referred to as “sign of admiration or exclamation” in the 15th century according to http://en.wikipedia.org/wiki/Exclamation_mark

I think it is just convention.

If you think about the = sign, you can argue it "marks" equality. On the other hand, if you use "exclamation sign", in my mind, the street sign pops up.