va_end is used to do cleanup. You don’t want to smash the stack, do you?
From man va_start:
va_end()
Each invocation of va_start() must be matched by a corresponding invocation of va_end() in the same function. After the call va_end(ap) the variable ap is undefined. Multiple traversals of the list, each bracketed by va_start() and va_end() are possible. va_end() may be a macro or a function.
Note the presence of the word must.
The stack could become corrupted because you don’t know what va_start() is doing. The va_* macros are meant to be treated as black boxes. Every compiler on every platform can do whatever it wants there. It may do nothing, or it may do a lot.
Some ABIs pass the first few args in registers, and the remainder on the stack. A va_arg() there may be more complicated. You can look up how a given implementation does varargs, which may be interesting, but in writing portable code you should treat them as opaque operations.