The argument is usually that it has limited effect when it comes to protecting the wearer. Simpler masks or home made masks can probably prevent transmission somewhat from the wearer by reducing the reach of droplets.

The argument against masks is based on 1) that people want them to protect themselves 2) that there aren’t nearly enough masks to do so.

So as I understand it, the authorities that say “don’t wear a mask” isn’t answering the question “would transmission be reduced if everyone wore a mask?”, but rather the question “Should I get a mask to protect myself”. Those are two very different questions.