TIME:1) The bot would play until the entire current map is cleared or enough trash is killed to spawn the RG. This is going to run your timer down to nothing and you will miss out of upgrading gems.
STUCK/INACTIVITY:2) After killing everything you want it to run back around to collect the globes after? Not only is this a time issue in a GRift, but what if you can't get enough globes in the current map? It's going to just run around until time runs out or your inactivity timer goes off.
EFFICIENCY:3) It's going to be faster to clear regular rifts for more keys than the time it takes to go through the process of backtracking and picking up globes in GRifts.
In short, this creates a lot of issues and would require a lot of time to get working, if it would ever work well at all (calculating pickup radius and avoidance settings, etc)... And it adds very little, if any, value.
Let's not forget that avoiding globes would also mean avoiding item pickup near them. This would mean the possibility of missed gear if your inactivity timer kicks off or you get stuck while backtracking. It could also mean un-timely town runs when they GRift clock is already running down.