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

At least allow mods to change titles. Even that isn't possible.

I mean whatever your concerns are, the admins/developers of Reddit are okay with this change.

where ok, or at least one was in a comment posted 'ages ago'. That's very different.

The reddit button for submitting a post still says "save". I brought it up to a reddit designer last year and apparently there's some mountain of technical debt that would make changing the label a week long task so its languished at the bottom of the priority queue for many years now.