Sometimes we can use a singular count noun without an article, in certain phrases like "at home" and "by plane." Almost always, however, a singular count noun does need an article or other determiner in front of it. Or, you might express a very similar idea with the plural count noun and no article.
For your sentence, you could use one of these instead:
"¢ The computer was invented in 1700. "¢ A computer was invented in 1700. "¢ The computer has been in existence since 1700. "¢ Computers have been in existence since 1700.
is wrong. The indefinite article in its generic function cannot be used in this sentence because in such cases the article has, as a rule, the aditional meaning 'any.' If we follow your advice and write
A computer was invented in 1700.
we'll create a sentence with a very strange meaning ? Any computer was invented in 1700.
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