Why can the rest of the world seemingly manage? US airports have rebounded and seem fine.
Not everyone has the option to stay home indefinitely. Even at the peak lots of had no option but to travel weekly - now nearly 18 months on, people have affairs which need sorting - people haven’t seen their families, some of whom may be sick or not long left or need help with their children etc. It’s not always Mr and Mrs Smith off to Malaga.
And have you seen the results of the day 2 testing? Barely anyone from Green and Amber countries has tested positive. You have more chance of catching covid in the queue at Tesco than you do at Heathrow.