In GarageBand, why does the damper/sustain pedal make the piano less bright?
Pianos in Garageband and Logic have separate samples for pedaled and non-pedaled keys, I do not think this is done by effect or anything like that. It simply reflects the sound of real piano when damper goes down.

In Logic you can edit the EXS24 instrument and replace the pedaled samples with non-pedaled ones if you want. That could work.