Or the Army is just stubbornly sticking with it because we have so many and a mountain of 5.56 ammo in stockpiles.
We're just lucky that we haven't been involved in a major war against a first-line combatant during that period.
Nostalga aside, the M14 was a piece of crap. The wooden stock swelled in high humidity, changing the zero. It was heavy. It was nigh on uncontrollable in full auto. It was supposed to be producible on existing M1 Garand tooling, but was not.
It served the U.S. Army as the main battle rifle for just 10 years. It took longer to develop the T44 into the M14 than the M14 served.
Had the trials been honest, we would have adopted the FAL, had a compatible weapon with NATO, and probably would then have used it for the next 40+ years.
There were major issues with the initial rollout of the M16, and they were not only well documented, but there have been many movies and books on the subject. And I’m sure you know the story: McNamara nixed chrome lining, the Army changed from stick to ball powder - changing the pressure curve and increasing fouling, and cleaning kits were deemed unnecessary and were not issued.
55 years later, the M16/M4 is not the jam-o-matic it was in 1965, when it was issued to troops in Vietnam with zero training and all of the above flaws.