@Abdo23, there are couple reasons.
1. There are two existing "Layout" tools out there that do this: Tiled (https://mapeditor.org) and Level Director X (http://www.retrofitproductions.com/level-director-x/). While these may not do everything you need, they exist and they work for many use cases. We don't feel we need to re-invent the wheel.
2. We have tried twice and both times we ended up killing the project, so we have some experience with this and it's not good. We have a pretty small engineering team and building a fully functional GUI would require multiple engineers working in tandem for a year or more. We would pretty much have to put a significant amount of our engineering capacity on hold to build this.
But with Corona moving towards open source, perhaps an enterprising community developer may want to take this on.
I appreciate you taking the time to respond. I'm not sure if what I'm about to say would be worth anything to anyone, but in any case, here is my own experience:
I'm not a pro programmer, I'm a designer, but I've been using Corona for the past two years to create my own projects. I keep fiddling around with Unity and other stuff from time to time but I always come back to Corona. Lately, I got an idea of a new game that requires some sort of a visual editor to design the levels. Knowing that Corona doesn't have that I decided to go with Unity. Not to bother you with boring details my experience there wasn't very good, yes it's powerful and easy to get stuff up and running, but I found myself struggling with several issues mostly regarding developing 2D games especially for mobile devices. I also had a look at Godot but despite its huge spike in popularity lately, I found it not to be very mobile friendly.
I then decided to go with Defold. While it's very powerful and keeps rapidly improving over time and already have a very active community for both providing help or making useful assets, I found it to be not mature enough and still needs some time to get some basics right. But I'm sure it's only a matter of time before it reaches that point. People there are working hard on it and keep improving it all the time. There might be some workarounds to overcome the issues I encountered there but as I mentioned I'm not a pro programmer and I didn't want to spend a lot of time and effort( and frustration) deciding how to come up with workarounds for these problems.
I then did some research trying to figure out where should I go next, and that's when I stumbled upon Tiled. I thought to my self "Great, Corona with a visual editor, I couldn't dream of anything more!" but boy was I mistaken. Working with Tiled itself was easy, the problems started when I tried to link everything back to Corona. The huge problem here is there is no official support between them by either of them. There is only a 3rd party library which had its own issues. When I decided to write my own code to overcome the weakness of that library I found that the exported file you get from Tiled is a complete mess beyond belief, I then realized that Tiled's UI itself is poorly done and gives false assumptions to its user and I've already explained those issues at their forums.
I've finally managed to ditch the 3rd party library altogether and wrote my own code that deals with Tiled's exported file to make it work with my needs. While some people might find this to be a fun process trying to figure out how to solve programming problems and such, to me it was the opposite, it was a necessary evil.
I love Corona, it does so many things right. It's powerful, easy, lightweight, easy documentation and the only thing out there that actually knows how to properly work with mobile devices. But at this day and age, the lack of a visual editor is just a deal breaker. Even though I like working with it, I still find that making stuff appear on the screen, positioning and organising them using code to be very redundant, especially when it comes to UI stuff.
I've decided that this will be my last project with Corona and I will definitely be switching to Defold as it does basically everything corona does (including the use of a very easy scripting language) plus the visual editor. But I would still strongly urge the people behind Corona to re-consider adding a visual editor to the platform, especially from a business point of view. Not only would it keep current users from migrating somewhere else, but would also attract newcomers and help grow the business. Just my two cents.