TLDR: Because they are the best at gaming the system.

Long version

They provide a really valuable thing to hosters: protection and reliability. The promise: "we take care that your website stays online".

Plus they have a very good caching infrastructure meaning my server isn't under as much pressure when I usee cloudflare.

For a user this is visible in two ways: bot testing stuff and slower load time, in individual perception.

And now the downside: they've created a spiderweb, sitting in the middle and most things online are entangled. "But I don't user cloudflare for my website!"? Tough luck. Some routing instance in between your users and your server might. The bit protection? Used to train various machine learning algorithms. Providing any kind of automation service online? If cloudflare marks you as something negative you're dead, no appeal.

They are from my assessment the single most powerful Internet company existing and they are staying nearly invisible on the public radar.

They pushed themselves into every problem people had with the more complicated layer in the whole Internet thing and made it easier for users - and leverage every information byte they collect on the way.

That said: I don't hate them; I'm just really miss trusting with this amount of power and influence centralized. They utilize the whole infrastructure to become a more engrained entity.