The selection of choosing the best possible timing positions has nothing to do with keeping the camshaft(s) stationary. If the camshafts stay stationary, it's usually because there is a split between the intake and exhaust lobes opening times called the Lobe Separating Angle or LSA. Depending on how this is setup, the LSA is usually between about 110° of crankshaft rotation to around 122°. This depends on a lot of factors to include things like whether the engine is turbocharged or naturally aspirated. This usually means, depending upon the number of cylinders and thus cam lobes, there's one cam lobe opening a valve and one closing a valve, which offset each other.
The cams timing position is designed into the cam when engineered. The angle at which it is set is prescribed by engineers to account for whatever performance variables are required by the specific application. In performance applications, there is the ability to change the cam angle to affect how an engine behaves. Move it one way and it will improve low end torque (TQ), like you'd want with a pickup truck or towing application. This is to the detriment to high end horsepower (HP). Move the cam the other way and it improves high end HP at the cost of low end TQ. You'd want this for performance applications where you're running your engine at higher RPMs like racing applications. Usually, under normal conditions, the manufacturer's engineers try to find the sweet spot between the two areas to give the vehicle owner the happy medium. In recent years manufacturers have been able to do this using variable valve timing, which, depending on the vehicle's needs, can alter cam timing on the fly and give a little better low end TQ while also improving high end HP.
Bottom line here is, the cam position isn't set to provide an easier time for the mechanic. It's engineered how it is for what is needed for the specs of the vehicle. The fact most cams stay put with the belt/chain off them is just a happy coincidence. I say most, because some cams move if not held stationary. Some multi-cammed engines are nearly (if not completely) impossible to set timing without some sort of holding fixture. They just won't stay still.