I mean:

On the one hand, it would greatly decrease the chance of ties, and the times would be more accurate.

But on the other hand... it would be more work... :(

The top times in 70 star aren't really at the point to where milliseconds are needed, when they get to 47 perhaps. For lower times in ALL categories this just isn't feasible because most people don't know how to accurately retime and end up just using split time milliseconds and at that point, what is the point? It would mean though that all mods would need to know how to retime, download every run and retime each one and edit the time on the leaderboards.