Because a very large proportion of the population has not been vaccinated.
The vaccines give very high protection but not 100% efficency. Efficiency varies depending on other health conditions. Two doses are needed for the highest protection. Only 66% of the population has had two doses. 72% has had only one dose. That’s 28% of the population with no protection. Almost 19 million people not protected based on a population of 67 million.
Around 8% of people aged over 65, those who are most likely to have co-morbidities, have not been vaccinated. That's approximately a million people.
There is evidence of vaccine-fade, vaccine protection becoming less effective over time hence the booster programme.
The high infection rate does not mean that everyone infected becomes seriously ill. Some people are asymptomatic, some feel a bit tired or have a cold.
The weekly number of deaths from Covid, people who died within 28 days of a positive test is now small, compared to the average number of deaths per week from other causes.
In August 2021, there were 40,460 deaths registered in England, 2612 in Wales (ONS data).
The leading cause of death in August 2021 was dementia and Alzheimer's disease and ischaemic heart disease. Covid accounted for 5.3% of all deaths registered in August in England and 2.7% of all deaths in Wales (ONS data).