SSDs are great, but the surrounding hardware also has to be optimized to properly utilize a SSD to its fullest potential.
If you use a SSD in your PS4, you'll get something like this (per Cerny during his presentation) -
Faster? Sure. But major bottlenecks still exist.
On the PS5 -
The reason the PS5 is exciting is because of the custom I/O that will reduce downstream bottlenecks and allow the SSD to be used to its maximum potential.