Yeah, but people aren't using Cloudflare just for DDOS Mitigation. Some are running pretty much everything over it, from DNS to edge caching to load balancing and even hosting. That's what I oppose mainly.
Unless you are really big, onprem stuff would be 90% internal anyway. For everything public you'd host your hardware in a datacenter with better high speed connectivity. And pretty much every single datacenter I interacted with in the last 5 years does have a DDOS protection solution that you can order for your network.
DDoS mitigation is one of those areas that an on-prem solution just isn’t well suited to solve.