[badsectoracula]

If a constructor is called automatically then it also needs the destructor to be called automatically and both introduce a bunch of additional complexity, especially for stack allocated functions, the need to take into account the concept of a default constructor (not necessarily requiring it but still needing to consider it), etc so that someone doing...

    while (something) {
        foo foos[42];
        /* use foos here */
    }

...will still have their 42 foo constructors and destructors called at the appropriate times.

Meanwhile by doing it manually you just bypass all that complexity at the language level and it becomes the programmer's responsibility to do it (or not) properly.

Also while you often want the above, this isn't something you always want, e.g. when you need to differentiate between where the memory of "foo" comes from and its lifecycle as an object (e.g. custom memory allocators, reusing objects without allocating/releasing their memory, etc).

Of course all the above could be done with functions too, after all there are a bunch of C programs and libraries doing OOP without the language itself providing any support for it.