Why do electrons flow from negative to positive, but current from positive to negative?

We don't always know (or care) what the sign of the charge carriers (the moving charges) is.

When the charge carriers are electrons, their sign is obviously negative. But charge carriers can be positive and negative ions in electrolysis, or a beam of protons (positive) in a particle accelerator, or electrons and holes (positive) in a semiconductor.

So we refer to the direction of the 'conventional current' *This is always taken as being the direction positive charges would flow if they were the charge carriers.*

This means that if the charge carriers are negative (e.g. electrons in metal) the physical current (the actual moving charges) flow in the opposite direction to the conventional current.

This is down to history. When Benjamin Franklin worked on electricity, electrons hadn't been discovered. He arbitrarily decided current was due to the flow of what we now call positive charges. That convention has stuck.

They guessed wrong about 3 centuries ago, and are still perpetuating the error, by calling it conventional current flow. In electronics and possibly ordinary conductors holes flow the same direction as conventional current flow.