Why do computers have only a few thousand instructions when they could have millions?
Because every instruction needs some circuitry to implement it.  The more instructions you add, the bigger the processor gets.  Since most of these operations will never be used, it's just wasted complexity.

All the complexity can even slow the processor down.  RISC processors, such as the ARM, were based on the idea of throwing away a large number of operations, and making the resulting smaller processor core as fast as possible.

Even modern X86 processors, with a huge instruction set, have ended up having a faster processor hiding inside, emulating a huge complicated one.