Dead battery after sitting for 20 days

I left my car in the garage for 20 days while on vacation. Upon return the car was electrically completely dead. This is the second time I've been stranded. After the first time I bought a portable charging device of the same make as road-side service had...just in case. However, thoughtlessly I stored the device in the wheel well. No chance of getting to it since there doesn't appear to be any way to release the lift gate in back. Further, the rep answering the phone at the Lexus dealership in Bellevue couldn't even tell me how to open the door. He was trying to get me to pry some plastic trim off. Turns out quite by accident the plastic levers that are under the door handles swing out and release the door latches when the electrical system is drained.

What I want to know is WHY has Lexus not provided a fix, either in software or with a floating battery that will automatically disconnect itself if the battery voltage drops below the needed amount to start the car? It makes the car UNUSABLE for anyone who leaves it for more than a few days.