Why isn't it the norm to have research repeated immediately by other academics?
Researchers make or break their reputation on ground-breaking, innovative, and high-impact work. Reproducing the results of another group provides essentially no benefit to any individual group (even if a culture of replication could arguably benefit the field).
More often than not, in my experience (in physics), reproduction of results is done as a first step in a new research project. The point of this is usually to check one's code/understanding before building up to something new. But it is not unusual for errors to be found and corrected this way. If a result is important enough to serve as a stepping stone for another research work, usually the key aspects of it are reproduced and it's not common for a visible, erroneous result to survive very long.