I think some of the "why" has to do with it being time-consuming and expensive. There's also the "hur-hur n00b, your spec can be wrong, so verification is point

I think some of the "why" has to do with it being time-consuming and expensive. There's also the "hur-hur n00b, your spec can be wrong, so verification is pointless" crowd that pops up any time the subject comes up. That said, my favorite verified software packages are seL4 (https://sel4.systems https://sel4.systems) and the CompCert C Compiler (https://compcert.org/compcert-C.html https://compcert.org/compcert-C.html).