There are other teams that try really hard to train their players not to fumble, including taking them out until they learn not to fumble. Belichick has been otherworldly good at this training.
The Patriots were not always otherworldly at not fumbling. In fact, this acumen started the same year the NFL changed the rules so that the team was allowed to have full control of the balls after pregame inspection.
This isn't a minor issue. Turnovers are hugely predictive of game outcomes.
The Patriots were caught with deflated footballs. They had deflated footballs for a reason.