DHE key exchange. Is it really secure?
A lot of modern cryptography is based on some mathematical assumptions and aims to achieve what is called Computational Security. That means that the adversary (Eve) could get some information about the plaintext with a negligible probability and the adversary is modeled as someone with bounded computational power, storage and bounded time. 
So all the (encryption) schemes are dimensioned in sort to achieve such level of security: that means that all mathematical elements are chosen to be large enough to make the best known attack feasible in a reasonable amount of time.

Even if this is a weaker notion of security, this assumption on the adversary is easy to accept in practice.

Other approach exists: Information theoretic Security or Perfect Security

So coming back to your solution based on a big table, if the scheme is correctly dimensioned the table you are supposed to use to attack the scheme would be so large that it cannot be stored in a (very very) large disk or to be (too) long to be generated.