I'm sure it is expensive to shut down and monitor roads in a city.
I did a DRT cargo race that was based in a central city park, but sent riders out like spokes in a wheel on a "scavenger hunt", open roads. And, I think there was at least one bike/auto accident. Nonetheless, it was similar riding to what I'm doing all the time now.
The "Crits" I remember as a kid, one was around a small park in Eugene, and another around a hill park, with a few roads closed, but nothing too critical.
Our local 24 mile loop TT is about 20 miles out of the city in nice farmlands. I assume they either close the roads, or do a lane drop, with only a few intersections to worry about.