__construct isn't just some special method: it's the constructor. By making it 
private you are disabling construction outside the class - __call can't be 
executed because the object can't even be created in the first place.

So now you think "Okay, well allow construction if there's a __call". But that's 
not an option because then any class that has a __call can be constructed 
regardless of whether the constructor is private. "Okay, then only allow it if 
__call handles the '__construct' method". But how is PHP supposed to know that? 
It can't. Plus, now __call doesn't know whether the object has been fully 
instantiated yet or whether it's executing before then, and that's a headache 
all by itself.

Next idea is making __call execute both statically and as an instance method, 
but that's not a good idea either: how is __call supposed to know when it's 
static or not? Don't forget there's __callStatic too.

And now we arrive at the idea of making a static method that acts as a special 
kind of constructor. Which is what getSingleton() does.

You can't override the creation of an object in that sense. No such feature exists. Hence, not a bug.