Quite a few platforms to support, editor tooling, store for game developers, and first party support from platform holders, before it can actually surpass Unity.
And Unity's ad network is in competition with several of these companies, most notably Apple and Google.
Not saying that Godot isn't going to get there, only that those that downplay Unity really don't get the spectrum of features it provides.
Google Docs versus Office.
The thing with AAA games is that they can afford to remove the Unity logo.