I don't think I'll find a better answer than Robin's:
[T]he main social function of academia
is to let students, patrons, readers, etc. affiliate with
credentialed-as-impressive minds. If so, academic beliefs are
secondary - the important thing is to clearly show respect to those who
make impressive displays like theorems or difficult data analysis. And
the obvious way for academics to use their beliefs to show respect for
impressive folks is to have academic beliefs track the most impressive
recent academic work.
So it won't do to have beliefs bounce around with every little
common sense thing anyone says, however informative those may be. That
would give too much respect to not-very-impressive sources of common
sense. Instead, beliefs must stay fixed until an impressive enough
theorem or data analysis comes along where beliefs should change out of
respect for it. Academics also avoid keeping beliefs pretty much the
same when each new study hardly adds much evidence - that wouldn't
offer enough respect to the new display.
Relative to the Bayesians that academic economic theorists typically
assume populate the world, real academics over-react or under-react to
evidence, as needed to show respect for impressive academic displays.
Still, I should point out that Robin has only explained, rather than justified, academic economists' deviations from Bayesianism. Anyone care to try to do that?