Why must data be copied from the hard drive to RAM in order for it to be used in processing operations?

Answer 1

RAM is accessible directly and at the individual data item level while the hard drive is accessible only indirectly after a variable length delay (depending on where the data is stored on the disk and where the previously accessed data is stored) and in blocks (e.g. sectors, clusters) that typically contain multiple different and sometimes unrelated data items.

Some old computers used hard disks (with hundreds of separate stationary heads) as their main memory instead of RAM, registers, etc. Such machines processed data serially instead of in parallel and were ponderously slow compared to modern computers. They were even slow compared to other computers of their day but usually cost less than a tenth of those computers and were both much smaller and lighter weight.