Personally as a beginner it's nice to have GDScript built-in. You can literally install Godot as a single executable and everything is self contained which vastly simplifies getting started versus Unreal where you have to install Visual Studio. (I have no experience with Unity but I'm assuming you similarly have to install some C# dependencies.)
As a programming language it's pretty reasonable. The built-in editor has auto completion and documentation, you can resume from errors. The syntax is basically Python with optional type annotations. Comparing the Godot vs C# examples in the documentation makes me shudder at how much more verbose C# is in comparison.
As others have pointed out you can use C# if you want to. They provide a separate download for it.
Well nobody forces you to use it. You can mix it with other languages, even more so in Godot 4 where languages can be GDExtensions.
You can write one script in GDScript and another in C# if you want.
I personally like to use it. It's not perfect but it's a language that's tailored for the task it's doing, namely driving the high level logic of a game engine.
And most importantly it's beginner friendly, so it's pretty nice for people who want to get into programming.
I'm a heavy-duty C++ programmer professionally, but I find GDScript really nice to work with. It's very well natively supported in the Godot editor, and it inherits the syntactic simplicity of Python that makes it very easy to write small programs in it without having to think much about the details of the language.
They should have used C#.