I don't think anyone argues that masks (at least non-N95 masks) stops viruses 100%. It's sort of a strawman argument. The opposing view is that if everyone wore a mask when in public places, we will reduce the transmission of Covid-19. The real debate is if this decrease is marginal as some are suggesting or if it is significant like most health experts suggest.
Consider condoms and STDs. Bacterial and viral diseases are still able to penetrate (sorry, couldn't think of another word) condoms. Nevertheless, 100% of health experts say that condoms help reduce the transmission of STDs. If someone was being promiscuous, it's terrible logic to say "Well, since condoms can't 100% prevent STD transmission, I'm just going to have unprotected sex. What's the point?" It's not very logical honestly...
[Now of course the best way to prevent STDs is abstinence, but that isn't my point. lol]
Well it's not a strawman because while it isn't explicitly asserted, it is being constantly suggested. Mask safety is suggested through statements such as, "Such and such crowd gathering was not dangerous, because they mostly wore masks, hence there was no spread".
And now when you mention STD's, yes this exact same thing, even worse, absolute effectiveness of condoms is being falsely suggested to people in the same way by constantly attaching the adjective "safe". Context is important.