SSDs are smaller, so you can pack more of them in the same volume. Not aware of really open designs (maybe in the OpenCompute project there are some), but just from a quick look at Supermicros homepage:

Something like https://www.supermicro.com/en/products/system/1U/1029/SSG-10... can fit 32 SSDs à 16 TB in a 1U system (Intel at some point announced 32 TB models in the same format, but I'm unsure if they ever were available).

This style can fit 48x 16TB in 2U: https://www.supermicro.com/en/products/system/2U/2028/SSG-20...

The trouble with such dense SSD capacity is more being able to interface them fast enough with the host and the outside world.