That's because they didn't have computers back
then.
Yes they did. They were the people who operated the calculating machines :-)
Altho I'm certainly no exspurt on the Queen's English, at least as far as
"correct" American English goes, the spelling for the word defined as
"People who are operating the computing machines" would be:
Computors.
^
This even holds true for people who stick 2 pieces of metal together with a
hot stick - Welders are the devices used, Weldors are the people running
the devices.
I've just checked the Shorter Oxford Dictionary (this is a 2-volume
English dictionary, mine is somewhat old). It doesn't contain Computor or
Weldor at all. A 'Welder' is one who welds, a Computer is a person who
calculates.
US usage may well be different (in fact the usage may have changed in
England since my dictionary was published, but I don't think it has).
-tony