Car batteries are VERY ill suited for deeper discharges. Remember their purpose is to provide VERY small, large bursts of power for the starting motor and initial electronic demands - then get charged right back up.
Due to this special purpose they have relatively small capacities. Once the car is running the alternator runs all the stuff and even provides power to recharge the battery. Think about it - most batteries in a car will be COMPLETELY discharged after about 30-40 seconds of running the starter motor.
Most decent sized car batteries would only allow for 4-5 charges of a 2200 3s pack before being nearly completely "empty". Some large trucks and SUV's have larger batteries, but most cars just have to supply 3-4 seconds of cranking power then they are done. So they make em to do just that - tiny crank and then get recharged.
Thus the reason for using deep discharge (marine or RV type) that are much higher capacity and are OK with being further discharged. This is needed for boats as most runs are short and you turn the engine off, then re-start, then off etc. RV's you sit and run lights and other accessories and they need much higher capacities - and more importantly the cells are used to being taken to lower voltages. That can kill a car battery in short order.