I will personally make this mistake very often too, but how hard is it really?

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

I hear you asking, what does it mean? 

HDD stands for Hard Disk Drive and SSD stands for Solid State Drive.

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

So an SSD is a 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 the past, now only used for archiving and for "third-fourth" files and programs because it is a very cheap medium. SSDs have dropped significantly in price to almost completely replace them, as they are much faster and more durable. 

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