This will blow your mind, but datacenters still buy tapes. It's just stupid cheap. In the future, chemical storage by DNA or something similar might play the same role for cold storage.
HDD is still the superior way to store data compared to ssd's. Ssd's are great for accessing your data fast but for people who have a lot of data they don't access regularly the reliability and price of an HDD is unbeatable.
As the other guy said, because they are way cheaper. I use them for media storage.
For 20tb of hard drive storage, you could expect to spend ~$400 (probably less these days), but the same price will get you a 5th that on ssds (maybe more these days)
If you are streaming video, hard drive read speeds are good enough.