Can you provide some data supporting those cases: "lots of them" as you say. The Government puts out a lot of information regarding detected infections, and I don't recall any highlighting passengers who shared an aircraft with an infected passenger and five days or so later developed Covid. There have been around 600 cases of infection detected when testing asymptomatic arrivals (air and land), so if this were a serious risk you'd expect quite a large number of fellow passengers to have become infected.
As well, since you're familiar with the numbers, do you know how many of the 100,000 or so travellers who've undergone two-weeks' quarantine developed infection during quarantine? And at what point in quarantine did they develop infection? Those are key points to understanding the effectiveness of mandatory quarantine.