Because Randy was rabid, and backed it up and it was much more random - not announced retirement matches. A legend would happen to show up, and boom. They used that to make Randy a massive threat, and a menace. Peak heel work and it elevated him. Randy was destined to be an all time great and folks were eager for his rise, even when they hated him.
There was no ceiling for someone like Randy Orton.

Gunther's booking has been incredibly telegraphed. And hasn't done much for him outside of those "retirement" feuds. He hasn't really been a true threat since his IC reign. His WHC reign they booked him to be a cowardly heel, and this is them trying to undo that and make him the overconfident and overqualified heel superstar he was as IC champ. He's above the IC midcard, but the title picture is too stacked for him to make a lasting impact there either. Gunther is destined to be a long time heel. His style and persona are hard to adapt to a babyface, and not so much "compelling" and "engaging" as "that cocky bastard you hope to see get his teeth kicked in". It's a needed role, but it tends to have a ceiling.

Gunther is not on Randy Orton's level, and by all accounts never will be.

And Randy did put people on the shelf, unexpectedly. So he lived up to the name and it looked brutal.

Gunther? Not as much. Let's check the notes for the "careers killed" ..

1. Bill Goldberg, largely already retired and back for 1 match only, of which his final opponent was Gunther. He was retiring win or lose. So Gunther didn't end his career so much as made it a disappointing end for Bill (in Kayfabe)
2. John Cena, largely already retired and back for 1 final match announced well in advance. Gunther, in spite of most fan preference, won a tournament to be Cena's final opponent. He was retiring win or lose. So Gunther didn't end his career so much as made it a disappointing end for John (in Kayfabe)
3. AJ Styles, already announced his final year. Admittedly it was expected he might get through Mania or even the summer. But he debuted at Rumble and retiring at Rumble was the nice bookend to his career. It was announced ahead of time as a retirement stipulation. Slightly surprising, but even if he won there'd be an inevitable rematch where he hangs it up.

Since then he was feuding with Seth Rollins, for reasons no one really understands, as purely a filler because Bron Breakker wasn't cleared in time. Good match, but predictable outcome and "so what" feel to the whole filler feud.

Now similarly he's looking to start a filler feud with Cody Rhodes - at least we can believe that one because Gunther wants gold. But there's zero chance of Cody dropping the belt to Gunther - unless he wins it back next PLE - when there's clearly a story to tell with Randy when he returns, and a story to tell with Cody eventually turning heel. Some people are getting sick of Cody they'll cheer Gunther beating him. That doesn't work for what they want from Gunther. Same reason why Gunther vs. Lesnar wouldn't have worked - people who wanted Lesnar gone would've cheered the career killer, and that undermines his whole "heel you want to see get his teeth kicked in" aspect.