I can think of a few problems with this just off the top of my head:

The first x minutes of a post are _heavily_ influential in the success of the post, editing after allows you to hijack successful titles for gain (spam)

For large subs, posts only hit the top of /new for x minutes (for small x), editing after is basically useless if it's not successful in that time

Mods patrol the /new queue as posts come in, allowing editing will complicate moderation which is already a difficult job