Because it abstracts security to something that can be done in the weeks before your flight, instead of vetting a person in the hour and a half beforehand.

Most travelers are repeat travelers. Exactly what vetting is being done for Pre that can't be done once in person that sticks with the person for 1-2 years and is renewable in a similar way as Pre?

Sorry for the slow response. Just found this comment.

The first and most obvious response is that tsa-pre requires your fingerprints, and tsa does not. That obviously leads to an amount of pre-vetting that is never capable of being done in tsa.

In addition to this, I'd compare tsa to a blacklist and tsa-pre to a whitelist. Perhaps if your estranged ex-wife's crazy uncle goes to the same bar as a known terrorist suspect then tsa would prefer you go through the additional screening (and thus deny you to tsa-pre). However, the connection to the uncle is only made once you apply to tsa-pre (as its a whitelist), and thus more vetting is done.