Because dedicated instructions suck up valuable encoding space, and the more instructions you have, the more instruction you have which potentially become obsolete with new advances in microarchitecture.

Not to mention that by sticking with simple single purpose instructions you make the CPU easier to teach to students. That is after all one of the goals of RISC-V in addition to creating a good ISA for industry.

Have we learned nothing about why we abandoned CISC in the first place? Those CPUs got riddled with instructions that never got used much.