The cheapest HDDs now start at under $0.015 per GB. The cheapest SSDs start at $0.03 per GB, with more desirable M.2 PCIe 4.0 models that don’t use QLC NAND starting at $0.06 per GB— about four times the cost of an HDD. Even though you could buy ten 2TB SSDs for $650, good luck packing ten or more M.2 drives into a single PC.
There are also more expensive HDDs, some potentially costing as much per GB as a cheap SSD. The WD Black tops out at 10TB and costs $240, for example. That might be fine for the capacity, but most HDD users focus on cost per GB over modest improvements in performance.
HDD technology isn’t sitting still. Sure, the first 8TB HDDs appeared in 2014, and like so many things, the rate of doubling has slowed down. Now, 8TB drives are ubiquitous, and 20TB models are readily available. There are even plans for 30TB, 40TB, and 50TB drives over the next few years.