why does a good battery keep dying?

Probably a parasitic drain , it takes intelligence and some attention to find it . If you want to go out in the dark and pull fuses look for any arcing or small spark .

For the battery to go dead that quickly something is probably left on like maybe a map light, underhood light, etc. A bad alternator diode can sometimes do this too. An accessory that isn't turning off can do this too like the air conditioner fan running on low speed after the ignition is turned off. The air compressor for the rear shocks on my 94 Fleetwood runs periodically to maintain the vehicle height even when the ignition is off. I disconnect the negative battery cable when I'm not going to be using the car. But since this is your daily driver you shouldn't have to do that. Just an example of what might be going on. Hope that helps! Jim