A man who was sacked by Bayern and would have ultimately not won the league with them and for the most part, hasn't been great as Germany manager.
Another man who's main achievement as a manager seems to be be playing good football and finishing 9th.
Both Iraola and Nagelsmann have thus far proven more in their careers than Carrick.
In Iraola's case, we at least have three seasons' evidence that he is a good quality PL-level manager. In Carrick's case, the only evidence of this is the (so far) 11 games he's been interim manager for us. Remove that and you're left with an unsuccessful three year stint in the Championship.
In Nagelsmann's case, any one of what did at each of his three club jobs (taking Hoffenheim from a relegation battle into the CL, taking Leipzig to the CL semi-finals, winning the Bundesliga with Bayern) would be the highlight of Carrick's managerial career to date, let alone having done all three while still being 6 years younger than Carrick.
I have zero issue with people preferring Carrick. But the idea that he is a
safer pick than these other options relies on you ignoring the fundamental risk in deciding you should judge Carrick on a handful of games as interim manager while judging other managers on multiple seasons of evidence.
And you're opting to do so not even because Carrick doesn't have multiple seasons of evidence as manager to judge him on, but because those multiple seasons were unsuccessful enough that you have to
hope they're less representative of his ability than his 11 games here. Which themselves haven't been flawless.
Or to put it another way: if one of Carrick, Iraola or Nagelsmann were to go on to not only fail as United manager but prove to not even be PL-calibre managers at other clubs, it would be overwhelmingly more likely to be Carrick. That bottom-end potential outcome exists with him to a degree it doesn't with most of the other managers on the list.
The primary sense in which Carrick is a safe pick is financial, as he doesn't require paying a release clause, won't command the sort of contract other managers would and won't attract competition from rival clubs. He's also available immediately and would start with a degree of fan/media support, which helps. But as an actual manager, he's a very risky pick.