Why is there a maximum charging current much lower than maximum discharging current on a li-ion pack?
Okay, let me offer this hand-waving explanation. After quick Google for [what limits charge rate for LiIon batteries?], several articles came up, like this one.
In simplistic and very general therms of Physics, I would say that there is an asymmetry in solid-state chemical processes during charge and discharge. Essentially the charge is a process of some "order" in lithium ions deposition. So a higher temperature works against this process and slows the ordering down. The discharge process is sort-off "disordering", and goes more intensively with increase of temperature. The Joule heat, however, warms up the Li-Ion cell in both cases. So the charge must go slower to get the final state of charge, while release of energy is not limited. That's why the asymmetry exists between recommended charge and discharge process for batteries.