Because that's "how it has been" for a long time. It is well known that if you want to change the content of a video, you gotta delete the old one and reupload

Because that's "how it has been" for a long time. It is well known that if you want to change the content of a video, you gotta delete the old one and reupload it. The reason google doesn't allow you to change it in-place is problaby to avoid "selling" of popular youtube videos to be replaced/injected with ads.