Considering how heavily policed the idea of the Grand MCU Plan is by fans, Kevin Feige and the Disney bots who possibly patrol Google (it's a JOKE, guys, stop sending lawyers), it's strange to note how many contradictions there are in the Franchise That Tony Stark Built.
Not only have we seen long-teased plans change at the last minute and planned projects and stories evaporate completely, but we've also seen retcons and conflicts in what things mean and where they were supposed to be headed. The latest in that particular line is the Mjolnir Issue.
In Avengers: Endgame, Captain America damn near made cinemas wash away with a tidal wave of glee when he picked up Thor's enchanted hammer and proceeded to smash seven shades of the proverbial out of Thanos. In the moment, nobody cared HOW this was achieved, because it wasn't important - we were seeing a stunning, perfectly crafted fan-baiting moment in action.
In that moment we didn't really need to think back to the contradictory scene in Avengers: Age Of Ultron that showed Cap almost able to move Mjolnir but not quite. After all, he had sort of moved it a little, so it was kind of seeded...
And anyway, we were told not long after that Steve could always fully lift Mjolnir, because OF COURSE he was worthy. He was the apple-pie eating, Hitler-punching, flag-wearing symbol of morality who had chosen to throw himself on a grenade and chosen to put his body under possibly fatal strain to become a Super Soldier and sacrificed himself to save the world and... Well, you get the picture.
Who cares if he bent some rules as he became more complex? Who cares that he was classified a war criminal? HE PUNCHED HITLER, he can lift whatever enchanted hammer he likes.
The Russos claimed on a Reddit AMA that Steve had merely been pretending in Age Of Ultron. That he didn't want to embarrass Thor by lifting his special, one-of-a-kind-bonded weapon.
"Our interpretation of the famous scene in Ultron was that when he realized he could pick up Mjolnir he quickly chose not to, because he didn't want to embarrass Thor."
It fit with the macho image of Thor at the time and his reaction to the movement, but it was always a little hollow. Like something of an ass-pull, in fact.
And more problematically, it would completely undermine Thor's entire arc, because Thor himself was made unworthy by Odin because of some... let's say bad choices. He couldn't redeem himself until those black marks were wiped from his playbook and thus he earned worthiness. So for Cap - who, by the time Age Of Ultron comes along is already harbouring a pretty dark secret thanks to The Winter Soldier and the revelation of Howard Stark's murder at the hands of HYDRA - to not be put under the same limitations by the enchantment is a stark contradiction.
No, Russos, he couldn't have just decided not to move it properly. He couldn't do it because of the lore of the MCU.
And luckily, the writers of Endgame seem to know that to be the case, as they told fans in the Endgame watch-along that Cap couldn't lift Mjolnir at all around the Ultron events. It took for him to wipe his own immoralities away (in Civil War) to get to the point of worthiness:
That's a far more fitting writing choice. And it makes Steve more interesting through the added complexity. When he was actively choosing to go against what he knew was the right thing to protect his friend, he was deeming himself unworthy. It was only when that stopped that he could hope to resolve the conflict.
Going further, it's the act of laying down your life - the ultimate sacrifice - for the sake of others that seems to suggest supreme worthiness. Thor did it and Cap did it once more in Infinity War (and then AGAIN in Endgame), standing up to Thanos when all hope seemed lost. That was where he earned those final stripes to be able to wield the hammer.
It wasn't just a matter of choice.