Reading List (Super behind but reading them nonetheless):
DC: Currently figuring that out
Marvel: Read above
Image: Killadelphia, Nightmare Blog
Other: The Antagonist, Something is Killing the Children, Avatar: TLAB
Manga: My Hero Academia, MHA: Vigilanties, Soul Eater: the Perfect Edition, Berserk, Hunter X Hunter, Witch Hat Atelier, Kaiju No. 8
He has been written that way the last 10 years or so. Although I don't think he was ever stated how he made the jump.
During Age of Ultron She Hulk and he debated who was stronger. In New Avengers while he punched Ben and Ben comments it was embarrassing way to find out who was stronger. Not entirely conclusive but indicates an upgrade to at least close to Ben
... I guess Luke is as strong as he needs to be for the story. I can remember the circa 1986 Marvel Universe roster book, it listed him as being 3-tons strong. The Marvel Role Playing Game published at the time listed him at "Incredible" which was a 10-ton maximum; I can recall the early 90s solo storyline where he was subjected to another chemical bath which reportedly increased his strength.. He went toe-to-toe with some heavy hitters during his stints in the New Avengers and Thunderbolts..
I just haven't seen Luke working out with one of those super-human weight machines like you see the Thing or She-Hulk or Colossus using.
I think Cage is actually a botch below Thing level, but he's able to hang with Thing level bricks through heart, the same way Thing is a notch below Hulk level bricks but able to hang with people above his weight class through heart.
I don't think there was an actual canon reason. He was just written that way. He's firmly mid tier now, above Spider-Man.
He hung with Proxima Midnight. They exchanged blows. He even had exchanges with Namor and other midterms.
Just something that happened I wouldn't bother looking for an in universe explanation.
The J-man
I have always liked Luke, even before he replaced Ben as the muscle on the FF.
Luke is a good man and I am glad he now has more power.
I think during a fight with Bushmaster, he was exposed to something similar to how he initially got his powers, giving him a power boost.
It was always pretty lame that a guy named Power Man only had like 1 ton strength. The power upgrade (or a name change) was long overdue.
also people overlook it but Cage and Ben have tussled in the 70s and it wasn't so one sided.
here's another encounter-
The J-man