I've never imagine someone would asking this question right on that YouTube videos. That would be funny.
It's all about entertaining the multimedia contents to make it more attractive. If they let the video without the helps of that supporters sounds, it will be a boring video with only the wind or the ball kicked sound, and the players screaming. They even cover the empty seats with interesting design and pictures.
That's how the games or tournaments now, to be able to still uplift the surrounding and still gain the vibes of having a audience especially if it will be televised, the event organjzers will put some background, designs, and music. It's quite boring to watch without sounds of cheering especially when it comes in gambling or sports tournament. Maybe it's the 1st time again OP got to watch events due to Pandemic.