Because I don't drive my car for days at a time and there is a significant load on the 12 volt battery that is always there. Think alarm system, data radio for myfordmobile, and who knows what else always has power. When the charge level gets low enough the car goes into battery preservation mode.
It only takes a couple of days without using the car for the 12 volt battery to drop to the 50% charge level.
If the car is driven at least 25 miles every day there is not a problem. That is sufficient to keep the 12 volt battery charged.
A run to the supermarket that is 2.6 miles away is not going to keep the 12 volt battery charged.
Keeping the car plugged in is the only way to keep the 12 volt battery charged in a days between drives situation.