Why is econ climb faster than econ descent?
In actual fact, sometimes it is not.
It very much depends on the individual airline, and how they want the 'econ' selection programed.
Case in point.
SV with L1011's and dual Hamilton-Sundstrand FMS's...the first widebody to have a true FMS, by the way.
Econ climb had several EPR settings and in addition, the speeds were 350 knots/M.85.
Cruise M.865, normally.
Descent (normally) M.85/350 knots.
Why? SV has very cheap fuel, so the airline wanted the fastest forward speeds.
OTOH, other airlines used slower descent speeds as their 'econ' selection.
As it is in most things in flying...sometimes it just 'depends'.