Because it's not the place of a DNS resolver to police the internet.
1.1.1.1 is simply a free DNS, 1.1.1.2 blocks malware, and 1.1.1.3 blocks both malware and adult content. It's a service that does exactly what it's supposed to do.
If I specifically choose a DNS server that promises to not resolve sites that will use my computer in a botnet, then it is that DNS resolver’s place to do that.
This particular revolver is an opt-in service for users that want Cloudflare to block anything that Cloudflare designates as malware.
Literally what the product is here.