Personally, I will also make this mistake very often, but how hard is it really?

It may be quite hard, but it is definitely not a disk.

I hear you asking, what does he mean? 

HDD means Hard Disk Drive and SSD means Solid State Drive.

The HDD contains large magnetic disks where data is written and they move with a motor when it is in operation. On the other hand, the SSD is in a "solid state", where data is written to flash memory, without the presence of disks.

So an SSD is storage space, it is a storage medium, it is a solid-state storage medium, it is Non-volatile Memory, but it is definitely not a hard disk.

HDDs are almost a product of yesterday, now used only for archiving and for "third-fourth" files and programs because it is a very cheap medium. SSDs have dropped enough in price now to almost completely replace them as they are much faster and more durable. 

Maybe we miss Hard Disks now and that's why we kept the name, maybe it's just a bad habit.