Acute (very short term) road closures are a double-edged blade. If it causes a traffic backup, the traffic system will detect it and route you elsewhere. Of course, for very busy roads with lots and lots of waypoints, it can take a little while for the routing engine to determine that there is a backup (and also when a backup has freed up), but I've found that even with any such lag, the Waze system for traffic reporting still seems far more accurate than anyone else in the industry.
If we let people manually report road closures, we run the risk of routing people around "false" closures if there are bogus reports. If we require multiple reports to try to avoid bogus reports, then we are back to what we have now.