I fully realize quarterbacks throw passes that are intercepted, receivers drop good passes, and that players and teams occasionally have a "bad" days, but, some games just seem to have a few too many interceptions and dropped passes, just enough to make one wonder. Overall, I think most players give the game all they have, because they want to win, but consider things such as the power the. NFL holds (as evidenced by the very slow acknowledgement of head injuries) & the amount of money riding on each games outcome,.
Although corruption exists in all sports, there is no large scale conspiracy to fix the outcome of NFL games. Referees are under a tremendous amount of pressure when deciding games. It is reasonable to assume that they will make a controversial call on occasion that leads people to speculate that NFL games are fixed. But the plays and signals of coaches are frequently changed in order to minimize the risk of game fixing.
The NFL works to minimize competitive advantages in its sport. It has rigorous drug testing methods, and has very stringent salary caps to prevent rich organizations from outpaying poorer organizations for the best talent. Cheating scandals in the NFL such as Bill Belichick's Spygate are frequently exposed to bring the competitive equality back to the sport.
In the history of the sport, there has never been any evidence of score or outcome-fixing. The contrary has happened in Major League Baseball, and there was Tim Donaghy, the NBA referee, who in 2007 was caught betting on games he officiated, but the NFL has never had a hint of scandal in this regard. The only claims of outcome fixing come from a supposed FBI investigation that was later dropped.
Most sporting events are fixed in order to create large gambling cash cows that cities can use as tourist traps. Those in the know are able to line their pockets by betting on the team destined to win while much cash is otherwise lost by people who believe the games are fair.