All the cold data is on spinning rust, and most of the data at large scales is cold. Much of the warm data is still on HDDs too, fronted by a flash or RAM cache.

The storage densities are constantly increasing to keep costs low. SMR drives have significantly pushed up the achievable densities at the cost of write performance which matters little for cold data. The 1PB/node mark was breached a few years ago.

Few players have the budget to store these volumes of data on flash, especially since you're always erasure coding for redundancy which adds anywhere from 50-100% storage overhead, depending on other tradeoffs.

HDDs are a remarkable invention and not going anywhere soon. Heck, I think tapes aren't dead either.