ATAGS cannot move hundreds of kilometres on it's own. It has a small auxilary engine that can be used for positioning.
Caesar does have an APU for power generation and a recovery vehicle to move it if it breaks down.
The truck that hauls ATAGS consumes more fuel and with two sets of equipment requires more maintenance. Caesar has proven so versatile it has not only been used and sold all over the world, we are even using it to replace our tracked artillery systems as well.
If you are facing an enemy with counter battery radar, you cannot leave your artillery in fixed positions. That only works for counter insurgency operations.
Caesar is only partially auto-loaded, if it breaks down you just do it manually like everything else.
That goes to the original point of the system weight.
Yes, ATAGS can move a few ten kilometres on its own at about 12kmph speed. Sorry for saying 100 km.
Sure, the truck that tows ATAGS consumes more fuel but the cost of the truck is saved. One truck can haul in several artillery. Also, fuel consumption of Caesar is high while operational as the Caesar is mostly automated and can't fire shells without the truck engine giving energy. Caesar may have APU but that is not enough to power it for long. Caesar can't be used manually as several things like the positioning of the barrel, the hydraulic lift to lift the truck are all automated without any manual ability. ATAGS can be used manually and hence can be fired till last shell is expended.
Next, Artillery is fired in bulk, not in a few numbers. So, shoot and scoot is not as easy as you tend to say. Shoot and scoot works in tanks, ATGM etc but in artillery, it is generally difficult.
There are auxiliary vehicles that carry reloads.
Are you sure it is less parts? There are plenty of parts on that gun, it looks more automated.
You have no more than 90 seconds from the time you fire to the time you need to be out of the zone. At the rate that APU moves it is not going to be out of danger.
If it isn't air mobile then it needs to be remounted to its transporter within 60 seconds.
The mobile artillery has its own use while advancing the position as the artillery can move with the team. But, mostly, artillery is used holding a position and causing massive destruction to opponent's position.
Towed artillery have much fewer moving parts that are compulsorily needed. The ATAGS does have some automation but all of them are optional. So, chances of breakdown or fuel being emptied in the middle of a battle is less.