That's actually one of the features of C#, if something is needed or is hot in another language, it will appear in the core libraries.
They've realised recently this is having a negative effect on open source C# code as so much of the community waits for the "offical" version, but it has its positives too, almost anything you want to do is in the core libraries.
This is my main grudge with .NET development, a culture of preferring The One True Way. It (among other things) leads to a lack of .NET FOSS diversity.
The biggest problem with this culture is that it means that when the One True Way is wrong, we suck it up and use it anyways.
I mean, Entity Framework is great, WebAPI is slick, and MVC has its moments, but there are some real trainswrecks. I've used all three XML serializers in the .NET framework and they're all cringeworthy. ASP.Net WebForms could have been something spectacular with open development instead of the hideous monstrosity it was. And MSBuild would have been laughed out of the room and regarded as some kind of bizarre eccentricity like TempleOS or UrBit instead of a serious build system.
>I've used all three XML serializers in the .NET framework and they're all cringeworthy.
XmlSerializer, DataContractSerializer, and what is the third?
I agree and was about to point out the same thing though; XML serializers in .NET are a joke but because they exist in the core libraries there is not really a compelling open source alternative.
That being said, it seems like the Asp.Net Web API is actually using Json.Net instead of the built-in serializer.
They've realised recently this is having a negative effect on open source C# code as so much of the community waits for the "offical" version, but it has its positives too, almost anything you want to do is in the core libraries.