Making sports is much harder than playing a sports, just as making just the right kind of rules is harder than following rules.
It seems that one could categorize most successful sports using just a few dozen formats. Standing. Walking. Running. Jumping. Kicking. Dancing. Riding. Swimming. Shooting. Object and body. Object and stick. Object and net. Etc. Perhaps it's a limitation brought on the definition of the word sport, but even still, the variations seem conceivable and any new sport is likely to feel derivative.
When was the last time a truly new sport was invented? Conversely, when was the last time a player invented a new way to play a sport? It seems that inventing inside a given set of rules is easier and more accessible than inventing the rules themselves.
Rules give players and onlookers a structure to measure success. Inventing an entirely new sport exposes the maker to unstructured criticism and uncertain comparisons. Furthermore, if the metrics for success are also new, then the players and onlookers will need to internalize them as well. It's no wonder we haven't seen a new sport in a while.
The world's oldest sports, polo, track and field, soccer and wrestling have lasted up to 3000 years following relatively simple rules.
There must be a lesson in here somewhere.