Because we let them do so.
If we stood up to them as Cleese is doing, they would vaporise under the scorching incandescence of truth.
The only weapon they have is glamouring, caterwauling noise. Period. They bully and intimidate, until the resistance overpowers them, and they will retreat.
Much of it IS illusion, but its a powerful one. People are generally wired to give into peer pressure. Think how many stories there probably are out there of kids getting into risky behavior like drugs etc because their friends pressure them. That's a pretty small amount of pressure compared to literally millions of leftards screaming waycism waycism at you. And what makes it worse is, at times, they have the means to harm people beyond just their nasty slander. Look what they did with Tommy robinson. It takes courage to oppose the crybully leftists and their mobs, and most people give in.
The best thing to do with a bully is punch them in the nose, which is a statement "I will not be pushed around without a fight" more people need to metaphorically punch the leftist crybullies in the nose. Every time someone gives in to these nasty leftists it gives them more power and more appetite to bully even more people. People need to learn to stand up to them, otherwise we will lose our right to dissent and become a totalitarian leftist society. which IMO we are well on our way toward, at least many western countries, certainly Britain is one of them.