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Why does the battery drain when the car is parked?

Why does the battery die when vehicle is parked?

Vehicles continue to use a small amount of battery power for the computer and anti-theft systems.

Additionally to what @Purebred posted, aftermarket accessories like anti-theft systems and stereos often fail internally and draw lots more current from the battery that the car actually would. Can draw the battery down overnight if it is bad enough.

How old is the battery?

If it’s 5 years old or more, replace it as preventive maintenance. It’s already on the downward slope. Some old batteries just don’t hold a charge, especially as the weather gets cooler

You might have significant parasitic draw. You should have 50 milliamps or less. Don’t let any fool tell you that 500 milliamps is okay, for example. It isn’t. Any competent mechanic can perform a parasitic draw test and, if it’s excessive, determine the cause