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.