Mere common sense? I wouldn't characterize it that way.

The wearing of masks to combat airborne disease spread is a decidedly not a new thing. That was never a novel concept, at least since the early part of the 20th century.

Coronaviruses in general aren't a novel concept either; we have a pretty good idea how they're spread.

So, when confronted with a new coronavirus, it's a little bit stronger than "common sense" to surmise that masks will be effective to some extent. The burden of proof would be on anybody suggesting that this virus is somehow so different than other airborne disease that the wearing of masks had no benefit.

    then why not let people decide for themselves using common sense. 

If masks are not effective, the downside to wearing masks was that we waste a relatively modest amount of time and money on masks and perhaps damage our credibility in the future when we recommend their use.

If masks are effective to some degree, the downside to not wearing masks was that millions of additional people would suffer and die worldwide, and we would incur a financial and human cost orders of magnitude greater than the modest cost of masks.

Because if you get it wrong, lots of people die, so you err on the side of caution.

Common sense comes up with the wrong answer here. The individual benefit was likely very small. The cumulative effect over many weeks on buildup in cases is much larger.