Why we still need Short Term Memory if Long Term Memory can save temporary data? If RAM is a short term memory and SSD is a long term memory, why don't microarchitecture of computer nowadays use SSD or another long term memory for saving temporary data like hidden variable for programming?
If it's about speed, then SSD can improve its speed, is it possible that SSD will become faster than RAM at some point?
If SSD has address for memory location and data for
opcode/instruction/operand like RAM, then will it possibly act like RAM?
There's two simple reasons, one fundamental and one related to our current technology. First the technical one: volatile storage is (generally) faster than non-volatile storage. It has fewer requirements - it only needs to store the data for a short while until it gets refreshed, so it's not a surprise that it often is faster.
But the fundamental reason is that memory gets slower to access the bigger it is. This is why modern architectures don't just have 'RAM' and 'disk', there's layers upon layers of increasing size memory, with only the topmost layer being non-volatile:

CPU registers
L1 cache
L2 cache
L3 cache
RAM itself
Cache on the disk micro-controller
The disk itself