Does your system use exclusive access? Historically, that was done for performance reasons, to avoid the relatively high cost at the time of copying live video to multiple destinations. Modern machines are easily capable of doing that now, but a lot of operating systems don't seem to have gotten that memo. Thus, even today, only one thing at a time can have the camera. Everything else has to wait until whatever already has it, releases it.
OBS intentionally grabs everything that your setup calls for as soon as it starts, and doesn't release it until it closes, or you delete the last reference to it. OBS does more than just stream and record, and so it may not necessarily know whether it's live or not. So it's always live.
