Because the better vaccines efficacy are ~95% (Moderna & Pfizer). This means that 95% exposed to the virus will not test positive but 5% of those vaccinated and are exposed to the virus will get the disease and test positive. Fortunately most of those will be asymptomatic and show no symptoms (and feel well), but others may exhibit some symptoms. Stats show that nearly all who show symptoms will have mild case due to the way vaccines prepare the body to battle new pathogens on entry. This is why the vaccines are great.
Unfortunately J&J and AstraZenica vaccines are less effective ~ 60% so most with these shots can still test positive if exposed and also spread the disease as well as get mildly sick.
For these scientific AND LOGICAL reasons, it is best practice to continue wearing a mask until most in a region are vaccinated with the better vaccines or the virus is eradicated in a region then all entering into that"clean" region are tested on entry. UK will have a longer battle to snuff out the virus as they are mostly AZ jabbed and only ~60% will be safe, the other ~40% of them will continue to spread the virus about among themselves unless they continue to wear masks to stop this.
The good news is that with ~80% vaccinated with the 95% efficacy, heard immunity will eradicate the virus in those regions. These models with shown over a year ago.
In our province, we had our active case counts to 0 for most of 2020 (with no one vaccinated) but our idiot PM (who controlled airports under FED Govt) continued to let untested people fly in with the virus. He is an evil ****er to have done this and the deaths and illnesses here are mostly due to him.