You're begging the question. Why should monks and priests be a model, rather than a reminder of human nature?

For the same reason I'd expect the pope to be catholic :)

Wouldn't you expect a full-time professional footballer/dancer/poet to be better at football/dance/poetry than the average person on the street?

How does one measure "better" when it comes to philosophy or spirituality?

The notion that priests and monks should be holier than the common folk strikes me as very Abrahamic. This forms a hierarchy in the mind.

I'm not a Buddhist, but if I were, I would interrogate (and probably reject) such hierarchies.

Buddhism is not some progressive movement to change the world.

Ethnic Buddhist traditions are usually among the most conservative forces in the society. They try to be conservative models. In Burma and Sri Lanka many of the politically most active monks favor ethnic cleansing and preach religious intolerance.