There's resistance in tango teaching and tango communities to the idea that followers can help decide how leading is done. If followers repeatedly accept invitations from leaders who take huge steps or shove them off to one side . . . well, no surprise, leaders will keep on doing that, regardless of whether teachers tell them not to. While it tends to be followers who tolerate poor leaders by virtue of accepting invitations, the core concept holds for leaders as well.
What "sticks" at the milonga isn't what's taught, elegant, polite, or popular in YouTube videos, but the laziest habits people can get away with yet still dance. For many leaders, that's minimizing the follower's space in the dance in order to show off cool steps or manly strides – or dancing with the follower off to one side in order to remove the need to learn to turn to see traffic and the need to learn to walk to avoid stepping on feet. For many followers, that's using the leader as a barre or pole to show off their cool ornaments and best floating princess impression, meaning they don't need to learn to manage their own energy and balance – or settling into an armpit because it's snuggly.
If followers and leaders simply refused to dance with people with those bad habits, said habits would fade away. But as those habits make dancing easier for many, they're tolerated . . . and newcomers see them and get infected by the same virus. Of course, those habits also make dancing less safe for all, but that seems to be a low priority for many. Those habits cut off entire realms of vocabulary, too, but that stuff would require more pesky learning anyway.
Improvement is hard, bad habits are easy. I strongly suspect whoever causes the armpit thing, it won't go away because it's too convenient for many.