Because humans aren't programmable either. As soon as you try to impose a complex program on a human, i.e. a set of high level instructions, you'll face a lot of complexity and end up with a process that's pretty far from what we call programming. Just think about whether you can program a programmer, or UI designer, etc.

Sure, you can program a human to do menial tasks and they can do it with acceptable accuracy but even that may require a lot of trial and error. ("Oh, but you said I should do this and that and never mentioned that in this special case I should do that other thing." Or, probably more relevant: "yes, you told me to do this and that but this situation looked different, so I solved it in another way I thought was better.")

>Because humans aren't programmable either

Apparently you have never met a human

Or heard of "brainwashing", "indoctrination", "education", etc

Humans are programmed all the time. All over the world. For all kinds of purposes.