If a build takes 4 hours it argues that a build on that system is a pretty expensive operation, unless changes are very infrequent, which in most systems is not the case, a build run on every change would seem to be highly problematic.
Also coz they don't want to add the queuing logic, so it takes 4 hours to run is an approximation. Also it messes with other systems so they prefer to run it overnight. This blog doesn't solve a real problem.