Ronaldo is, statistically, a very mediocre free-kick taker. He takes them because he is in charge, hierarchy. Mata is one of the most respected players in the squad and is a proven free-kick taker at the highest level, so no, it isn't necessary to be alpha but you have to be high up in the ranks, which Mata is. I can't remember Fred taking a free-kick but okay. As I said, there's no coincidence that Bruno and Rashford take them. You got to wonder why Lindelof (a freekick taker at Benfica) isn't even trying. Or Greenwood, who has scored at youth level, just walks away. Both Bruno and Rashford, and now I'm being kind, is highly mediocre at taking them.
Your idea of ‘mediocre’ must be compared to the best in the world. But the hierarchy of free kicks is based on comparison to the best freekick takers in the team.
Ronaldo have not been mediocre in comparison to the players he has taken free kicks ahead of. Neither has he taken all free kicks at United nor Real Madrid nor Juventus.
Mata took free kicks as a young kid at Valencia, and as a fresh squad member at Chelsea, not because he was one of the most respected in the squad, but because he was a proven free kick taker first in training, then in games.
Lindelöfs free kicks history in Portugal is highly inferior to Bruno’s. Incidentally, Rashford has scored as many free kicks against Benfica as Lindelöf scored
for them (you can guess how many).
Free kicks seldom result in direct goals, and part of the reason Ronaldo and Rashford get to take as many, is that their blend of power and technique makes freekicks from 25 yards sometime lead to rebounds and chaotic situations. This made Rashford take most free kicks at United at a point when he was far from an ‘alpha’ or ‘established hierarchy’ in the team.
I can guarantee you that if Mason nets 2 out of 10 regularily against David and Dean in training, while Marcus and Bruno does 1 out of 10, Mason will take free kicks in games, and a lot of them. The same goes for Amad, when he starts accrueing regular playing time.