Because 1% of the posts that resurrect an old thread might actually have some useful information -- not for the OP but for someone just dropping in.
80% cause problems by
1) Hijacking the post
2) Posting bad code requiring people to correct it
3) Posting wrong code requiring people to besmirch/correct it
4) Uselessly reiterating what was said before
5) Stating an option that is not usable
6) Starting arguments
7) Infracting new members
The other 19% are worthless and sometimes are just ignored, except by those that have to point out the resurrector is a dummy.