It's because OBS Studio does not separate the audio track files and the video track files but renders only 1 single video file w/ audio; and if you're recording 4K Videos for youtube, you want both video and audio separate for post production (Video Editing) for jump cuts, B-Rolls, Etc. also, you want to be able to edit the Audio, normalize, use Compressors to make the quieter parts a bit louder/clearer while the loudest parts a bit quieter to have a more balanced audio overall. Not to mention, loads of Background audio(music), etc.
I have a tiny Youtube channel and this is one of my issues with OBS, it doesn't let separate the video & audio tracks for my Post Production team to work on them. This is of less importance for streaming though.
Imagine recording 4K Videos that are 10GB - 50GBs and you have to use the same Video Files just to edit the audio, it would be so wasteful.