It's difficult to explain when you have never played any competitive game or actually faced like tough matches during school/college days.
Competing is the basic human nature,greater the struggle more the excitement.
And majority people want competition, that's why you will find huge crowd when 2 heavy weights are competing ,which means hard match.
This is true in all sports , boxing , car race, footbal etc.
No 1 cares about match between 2 teams where it's already known , 1 team will fold and other will win.
just like I didn't watch India / afganistan match and only saw highlights. And that's why whole stadium was also empty.