They’ve been separate desktop environments from the start. From top to bottom they share nearly nothing. The compositors, window managers, toolkits and shells are all different.
They also are ideologically opposed. If they merged, which direction would they go? The more feature-rich KDE? Or the more streamlined Gnome? Such a merger would lead to infighting and stagnation.
This is before even talking about the actual code underlying both environments.
I think it’s better for everyone if they stay as two separate projects.
How did it take them 80 years to find a bomb in a back yard? Is it a large yard? Was the bomb buried?