Because people are passing it within households when they gather with friends and family. Overwhelmingly that is where/how it is spreading
You don't know that and even if true, where are they getting it from???
Nothing to do with people sitting in and around pubs restaurants, in close proximity to each other...... or people with symptoms, unable to test and continuing with their daily lives....
The Govt message is that its all our fault, when in fact its theirs, failure to test, track and isolate, advising us all to stop WFH and go back to the office, use public transport and most bizarrely "eat out to help out"
Viruses spread through increased social contact and Johnson has encouraged us to increase our socialising since june/july.