I guess it is because they need to have a long career in politics to appear experienced. Well, they really do need to have experience to ratfuck everyone else and to build connections because particular politicians are a tools to ruling class and it don't really care who is the one doing its bidding, so politicians do need to compete among themselves. Also the more ossified bureocracy is, the more ancient cadavers are getting propped by the vitrue of having more accumulated ties inside the bureocracy than the younger ones, that's why US has Biden now and that's why catholics had most of their popes.

Also name is a brand. People, especially content liberals and conservatives, are more likely to vote on someone they always heard than someone new.