Because of the phantom loads that are on each set of batts. The dash radio, the memories in both the engine and the transmission, the connections to the CO and LP detectors, the steps, and many other devices. Sometimes the manufacturer will directly connect something that draws current from the batts just because they don't want to pay for an extra 6 feet of wire.
This is all despite whether the shut off switches most RVs have are on or off. If they are on, the batts will die faster of course.
What people do is just disconnect the grounds from the chassis batts, and remove the jumper between the 6 volt House batts. After they've been serviced and when they are at full charge...and you're storing the RV. You can just use the switches for 3-5 days for old batts, or 5-7 days for brand new batts and get away with it though. Try to go longer than that and most older RVs the batts will be dead when you come back. Some fancy schmancy RVs have switches or relays that do completely isolate the batts. Yours isn't one of them.
Now if you are plugged into shore power, and the chassis batts die, then you need a float charger from Walmart ($25) connected to the chassis batts OR you can pay $70 for a Trik-L-Start device...which is what I have because I don't have a nearby AC outlet in my RV basements and don't wanna run an extension cord outside on the ground.