Earlier quoted context omitted.
Because a vendor will do a lot of ground work (choosing a CNI and CSI implementation for instance) for you, and everything usually covered by a cloud-controller will be entirely up to you (e.g. LBs)
Actual bare metal, where you own the physical hardware and pay for the physical network connections is actually pretty painless. I see many people trying to use hosted compute to try and set up a bare metal cluster. This is a recipe for heisenbugs.
Having physical access (or IPMI) certainly helps, but there's also a lot more knowledge about networks bundled in companies that already run data centers, so setting up something like MetalLB (BGP load balancers) and Rook (Ceph CSI) to cover the parts that your cloud vendor would usually provide automatically is not as big of a deal. But the overall complexity for someone completely new to the topic is still higher.