Southgate had no experience at coaching at any level when he took the Boro job.
Viali was player/manager at Chelsea.
If we count interims, there's loads more.
Yes, but they had some kind of notoriety in football.
Kieran McKenna does not spring to mind for a premiership football club. Or even a championship football club, quite clearly. That's why the team that is knocking is Ipswich Town and his realistic chances of a premiership job were astronomically small. Yes it's possible but the chances are a lot closer to 0 than 100% likely no matter how long he coaches United for.
If you look at these examples and try to work out why they might have been appointed there is often a romanticism/reputation as a player that makes the appointment palatable to fans and marketable. Or some kind of experience under a genius manager combined with playing experience that leads to hopeful optimism said manager is the second coming (Arteta).
There is usually some kind of reason a chairman makes the decision he does even if it proves to be completely erroneous. They don't just produce a Kieran McKenna out of a hat at Prem level.