Why the hell doesn't unity give an HTTP post API so that the byte array buffer can be reused?

As the title speaks.

 

Because it's built into Mono.

 

Technically you can just create your own UploadHandler I suppose, or use Put instead if you don't absolutely need POST

https://docs.unity3d.com/ScriptReference/Networking.UnityWebRequest.Put.html

 

My point is that unity API always exposes a byte[] parameter, but never an offset and a length.