A stack frame has a fixed size and is deallocated when your function returns. So it's pretty useless for dynamic allocation. The size of a stack frame depends on the code at compile time so you can't dynamically allocate a stack frame anyway.

Stack is where you store your local variables and the heap is for longer term memory usage. A heap allocation can survive many function calls. If you allocate heap memory using a local variable for the pointer then you have to store that pointer somewhere so that you don't lose track of the memory when the function returns and the original pointer is lost forever, which would be a memory leak.