People suggest wearing masks for two reasons.
1) To stop the wearing being infected by other people.
2) To stop an infected wearer spreading their infection to other people.
Experts recommend against masks because:
When correctly put on, and worn, and taken off there's some protective value to masks if they're combined with all the other PPE.
Correctly putting on PPE, and correctly taking it off, is hard. Healthcare providers employ staff to act as "spotters" or "dofficers". This person's main job is to help people put on and take off PPE.
If you're wearing a mask by itself you get almost no benefit. You lose all benefit if you're putting it on wrong, or taking it of wrong, or wearing it wrong.
As well as that we see that masks increase risky behaviour. People wearing masks feel that they don't need to maintain 2 metres social distance. It's hard to speak through a mask and it's hard to hear someone talk through a mask so people both speak louder (generating more droplets) and move closer (increasing the risk of contamination).
People who advocate against masks look at the evidence which shows marginal benefits from mask wearing and they look at all the things which reduce or negate those benefits and they say that the balance tips away from wearing masks.
Whether you wear a mask or not you need to keep 2 meters away from people, so what does wearing a mask get you?