Because literally just one person being infected in that 60,000, assuming an R0 of 2.5 for instance (most estimates have it as being between 2.2 and 2.8), would end up being the index case for approximately 10,000 cases.
We have in fact seen a very similar case already in South Korea where the 31st patient they identified failed to follow the isolation procedures and has infected a huge number of people indirectly though her actions.
You can say 'who cares about 10,000 cases in a country of 60,000,000? Well, with less cases confirmed so far in the UK, we have already had to cancel all elective surgeries, scale down outpatient services, go into war footing, pull other specialty doctors into the medical and intensive care units and some intensive care units (Hillingdon/ Northwick Park in London for instance) have already started overflowing into theatres and turning down other patients who may have gone to ICU.
So you can disagree with the measures, you can disagree with how people are coping. But it is false to say that one event holding 60,000 people will have had little effect to the infection rates in this country.
Others have alluded to this already and I'm not directing this at you specifically. It is totally OK to be curious, totally OK to be reading up on it and totally OK to have opinions on everything in life, including this. Why so many people are insisting on taking their often misguided opinions and presenting it as fact though I have no idea.