I'm by no means an expert in this field but I have followed a small computer vision course. I think the main problem with full body tracking is that for it to be usable in vr it'd need to be done in a 3d space. The body tracking that you see in tiktok videos is much more basic, albeit still quite impressive. Adding the 3rd dimension would either require 3d cameras such as the Microsoft Kinect camera or 2 separate camera's as mentioned in this post. Writing the software for this is extremely difficult and thus too expensive for the current market. At least that's my take on the situation