HPC is almost entirely "commodity hardware" on the CPU and GPU side these days. Meaning there is no problem for someone with a bit of money and time to get one

HPC is almost entirely "commodity hardware" on the CPU and GPU side these days. Meaning there is no problem for someone with a bit of money and time to get one node (~ one motherboard with all its associated parts) equivalent to a top supercomputer. As others have said, it is the interconnect that makes HPC special. The ability to run a job across tens, hundreds, thousands of nodes, where all nodes need to exchange large amounts of data with all other nodes every millisecond, without network being a huge bottleneck that leaves your CPUs idling. If your job is never big enough to outgrow the biggest and baddest single server money can buy, then indeed you don't need HPC. But if you are running e.g. a full fluid dynamics simulation of an F1 racecar, a wind turbine, or a jet airplane, you sure do.