Actually we could. Maybe we won't do such studies because they are irrelevant because avoiding spitting into the surgical opening is a reason good enough. But then we cannot claim that masks during the surgery prevent the spread of airborne viral diseases.

But if we needed to guard for them and the good evidence is lacking, then not testing would be unethical.

Yes, even covid vaccine today could be compared with placebo, for example, in children. Europe never mandated covid vaccine for children and today in the UK they cannot even get the vaccine unless in a risk group. The US however recommends covid vaccine for children without the evidence that it makes any difference today. It definitely should be tested in trials before such recommendations.

I don't think that IRB would reject such studies. At the start of pandemic everybody was saying that doing human challenge trials by infecting healthy volunteers would be unethical. And yet the UK did them. The red tape takes time and I can understand that during pandemics we may need to act quickly and cannot test everything. But in principle we can and do need to all kinds of trials to obtain proper evidence.