Like many "can't stop" claims, there is ambiguity here that muddies the water. Do you mean "can't 100% stop" or "can't ever stop"? Of course, nothing is 100% when it comes to protecting yourself from COVID-19. Masks effectively reduce the chance of contracting the virus or transmitting it to others.
A very large 6-month randomized-control-trial (https://www.poverty-action.org/publication/impact-community-masking-covid-19-cluster-randomized-trial-bangladesh) involving 600 villages in Balgladesh and 350,000 people has settled any lingering question about the effectiveness of masks. They work.
"Among school districts in the greater Boston area, the lifting of masking requirements was associated with an additional 44.9 Covid-19 cases per 1000 students and staff during the 15 weeks after the statewide masking policy was rescinded... which corresponded to an estimated 11,901 cases and to 29.4% of the cases in all districts during that time." (2022-11-09)
Looking back to mask policies in 2020: "When compared to no mask policy deployment, mask policies might reduce incidence growth by 13.5% to 17.8%... The mask policy is effective in controlling illness, according to the bulk of the data shown above. This result confirms the mask policy’s importance as a governing approach in the context of the worldwide pandemic." (2022-11-05 PREPRINT)
"Our estimates imply that the mean observed level of mask wearing corresponds to a 19% decrease in the reproduction number R." (2022-05-31)
(schools) "Districts that optionally masked throughout the study period had 3.6 times the rate of secondary transmission as universally masked districts; and for every 100 community-acquired cases, universally masked districts had 7.3 predicted secondary infections, whereas optionally masked districts had 26.4." (2022-05-20)
"We matched exposure information from COVID-19 case investigations with reported test results and calculated the secondary attack rates (SARs) after masked and unmasked exposures. Mask use by both parties reduced the SAR by half, from 25.6% to 12.5%... Masks significantly reduced virus transmission when worn by both the case-patient and the contact." (2021-10-12)
"A randomized-trial of community-level mask promotion in rural Bangladesh during COVID-19 shows that the intervention increased mask-use and reduced symptomatic SARS-CoV-2 infections." (2021-09-01)
"Mask wearing and the establishment of COVID-19 zones for infected residents were the two main covariates associated with lower secondary-attack risks... Wearing masks and isolating potentially infected residents appear to be associated with a more limited spread of SARS-CoV-2 in ACFs." (2021-06-21)
"We find that most environments and contacts are under conditions of low virus abundance (virus-limited), where surgical masks are effective at preventing virus spread. More-advanced masks and other protective equipment are required in potentially virus-rich indoor environments, including medical centers and hospitals." (2021-05-20)
"Nonmedical masks use materials that obstruct particles of the necessary size; people are most infectious in the initial period postinfection, where it is common to have few or no symptoms [references in original]; nonmedical masks have been effective in reducing transmission of respiratory viruses; and places and time periods where mask usage is required or widespread have shown substantially lower community transmission." (2021-01-26)
"A 10% increase in self-reported mask-wearing was associated with an increased odds of transmission control... communities with high reported mask-wearing and physical distancing had the highest predicted probability of transmission control." (2021-01-19)
"We use the synthetic control method to analyze the effect of face masks on the spread of COVID-19 in Germany... we conclude that face masks reduce the daily growth rate of reported infections by around 47%." (2020-12-03)
A systematic review and meta-anaysis of 21 studies found "Use of masks by healthcare workers (HCWs) and non–healthcare workers (non-HCWs) can reduce the risk of respiratory virus infection by 80% (OR, 0.20; 95% CI, 0.11–0.37) and 47% (OR, 0.53; 95% CI, 0.36–0.79)." (2020-09-15)
"Universal masking at MGB was associated with a significantly lower rate of SARS-CoV-2 positivity among HCWs... these results support universal masking as part of a multipronged infection reduction strategy in health care settings." (2020-07-14)
A meta-analysis of 172 observational studies found "Face mask use could result in a large reduction in risk of infection (n=2647; aOR 0·15, 95% CI 0·07 to 0·34, RD −14·3%, −15·9 to −10·7; low certainty), with stronger associations with N95 or similar respirators compared with disposable surgical masks or similar." (2020-06-01)
"[most] fabrics remove up to >90% of the viral load on <10 µm particles in humid exhaled air if leaks are avoided, but modeled leaks around the nose dramatically reduce mask performance. As a consequence, when leaks are present, thinner and more breathable fabrics can outperform thicker materials which are less permeable and so force unfiltered air through the leak." (2022-05-19)
"In general, while non-medical masks can help prevent the spread of COVID-19, medical masks and respirators provide better protection. No matter which type of mask you choose, proper fit is a key factor in its effectiveness."
This study in Science reconciles the seemingly contradictory evidence that "regions or facilities with a higher percentage of the population wearing [mostly cloth & surgical] masks have better control of COVID-19." compared to studies that show cloth & surgical masks offering only marginal protection. (2021-05-20)