The main thing is that p100s tend to be elastomeric respirators, and those have a substantially better fit than traditional n95 respirators. I’m not going to go digging for the study right now (should be easy to find) but even heathcare workers who had previously fit tested with an n95 only had a bit over 50% chance to get a good seal in a realistic scenario, and it was far worse for people who had not fit tested. Every person who used the elastomeric respirators got a good seal.

Most people don’t do at home fit testing, and even if they do they aren’t testing in a real world situation where they are wearing the respirator for a long time, turning their heads, etc. Elastomerics are just enormously better in these situations.

A pre-covid randomized clinical trial that looked at whether N95s provided any meaningful additional protection against another aerosol-borne virus (flu) to healthcare workers working in outpatient settings found no benefit:

https://jamanetwork.com/journals/jama/fullarticle/2749214

Question: Is the use of N95 respirators or medical masks more effective in preventing influenza infection among outpatient health care personnel in close contact with patients with suspected respiratory illness?

Findings: In this pragmatic, cluster randomized clinical trial involving 2862 health care personnel, there was no significant difference in the incidence of laboratory-confirmed influenza among health care personnel with the use of N95 respirators (8.2%) vs medical masks (7.2%).

Meaning: As worn by health care personnel in this trial, use of N95 respirators, compared with medical masks, in the outpatient setting resulted in no significant difference in the rates of laboratory-confirmed influenza.

One person's cough may emit 10k aerosol particles. Another person's cough may emit 300k. Normal breathing also creates aerosol particles. And they can remain airborne for hours. Only one virion needs to make it through to initiate an infection. And nobody is wearing eye protection anyway and you can get infected with the flu or covid or any one of a wide range of other pathogens via the mucus membranes around your eyes.

I know that it seems as though masking ought to help reduce covid infections (and respirators definitely greatly reduce exposure to, for example, harmful dust particles for home renovators and industrial workers), but it doesn't. Yes, I know many experts and officials persist in claiming otherwise. They are wrong and either ill-informed or lying.