Why is there still no Edit Town/Neighborhood tool in the Sims 4?

Do you mean being able to place objects outside the lot? We have seen that it's possible with TOOL and it doesn't look like it's breaking saves. I think that's just a question of priorities and budget.

If you mean being able to remove things in the neighborhood, like houses or trees, then I think the bugs that players have seen in Evergreen Harbor are an indication why that would be difficult, it could probably result in the ground disappearing or break saves.