Holo specifically has it so they can delete yab later. They had manager watching and editing vods all the time, even more so on handcams where reflections and shit can be found. Since they are the biggest, people just kinda ... copy them without thinking why Holo does it. And eventually it became vtubing standard practice. I highly doubt Phase/Nijisanji has a legion of managers on the ready to took down vod the moment the stream end then edit them like Holo.
That being said, you can enable playback/rewind even if they don't have it enabled
using firefox scripts. I installed this one and can confirm it 100% worked. Idk about chrome.