Well that they sort of do have, but its in the form of the World Timer Aqua Terra which comes in at 14.1 mm, reasonable and only 1 mm more than the regular Aqua Terra but still not exactly slim for a watch without a movable bezel. 14.1 is ok though imo.
The issue with this is two-fold though, the first is that the worldtimer complication once you understand it is just as good or even more useful than a true GMT bezel, but it just isn’t well known or understood, people looking for a GMT watch won’t think to consider or find in searching the world timer, despite it being perfect for them.
The second issue is that it hasn’t been marketed well at all, it’s not a watch that even comes to mind for most Omega owners and nothing is being done to popularize the idea of a world timer as an alternative to a GMT watch.
This does give you everything the GMT bezel does with less work though as long as you know which city is near the city you’re in or at.