Conservative is the name of the game here, and its not specific to banking, whole society runs like that and you would struggle to find a company behaving differently, mostly just startups.
They simply prefer stability over bleeding edge, and job hoppers are anything but stability, that's obvious from a light year away to anybody with any experience. In fact it signals a person who is often not happy and instead of improving things they just move - one of the trademarks of juniors with big egos.
Also hiring on a perm position takes time and effort (and costs a lot), nobody is eager to waste so much resources again and again just to have potentially slightly more efficient employee.
I've been on the other side and agree with arguments generally, but this is generally their way here and they are doing just fine.
As for senior roles, you have your data backwards. Senior roles have higher turnover, including in Switzerland. They're paid more, but there is also much more risk associated with the roles. They're some of the first roles to go during strategic and structural shifts.