I do think it would be good to have this feature though with everyone starting a certain amount of time behind the previous starter.
Not sure why Mark Hewitt comment got a -ve vote though as the current system of races does not suit TTing.
Having none racers on the course confuses things as it can be hard to monitor your competitors when you are moving through a large group of non racers.
If you are in a small breakaway (1 or 2 others) it can also be hard to drop other riders if they are able to take advantage of the drafting of non racers.
Also, not everyone has the TT bike unlocked. To therefore do TT on Zwift you would need to ensure no one drafted anyone. The only way to do this would be to have a clear road.