Why must base class be constructed before derived members?

[Ganado]

I ran into this problem when trying to encapsulate behavior into an existing, rigid structure of a project I'm working on.

See both the DerivedBad and DerivedGood examples below:

#include <iostream>
using namespace std;

class Base {
public:
    Base(int foo)
    {
        cout << "Base, foo = " << foo << "\n";   
    }
};

class DerivedBad : public Base {
public:
    DerivedBad()
      : fab_(43),
        Base(fab_)
    { 
        cout << "Derived, fab = " << fab_ << '\n';  
    }
    
    int fab_;
};

//------------------------------------------

class BasePrereqs {
protected:
    BasePrereqs(int fab)
      : fab_(fab)
    {
        
    }
    
    int fab_;
};

class DerivedGood : public BasePrereqs, public Base {
public:
    DerivedGood()
      : BasePrereqs(43), Base(fab_)
    {
        cout << "Derived, fab = " << fab_ << '\n';  
    }
};

//------------------------------------------

int main() {
    
    cout << "Bad (UB):" << endl;
    DerivedBad d1;
    
    cout << endl;
    
    cout << "Good:" << endl;
    DerivedGood d2;
}

Output (perhaps):

Bad (UB):
Base, foo = 86
Derived, fab = 43

Good:
Base, foo = 43
Derived, fab = 43

DerivedBad is bad because it suggests that Base is being constructed after fab_, but in reality, the Base class is always constructed before any derived member variables, so 'foo' is uninitialized.

My question is: why? What advantage was there to designing the language like this, vs. giving the user the freedom to construct Base when they wanted?

I guess it's more philosophical, but I'm wondering if there are any concrete edge cases this restriction solves, or similar. I feel like I shouldn't have to make this "prereqs" base class just to force a certain order of construction.

(Probably the biggest issue I have is that DerivedBad even compiles! But that's a general issue with C++ outside the scope of this post.)