Ignorant people leave off the “d” in served because they have heard the expression without trying to understand what it means. The meaning is simple. My conclusion is simple as well. Those who say it wrong and write it wrong are just plain stupid.
The first person to come is the first one to be served.
Correct spelling requires more than the letter “d” in the expression. The correct way to write it is with hyphens: first-come, first-served.