> Math is the most formal science there is.

That's assuming you ascribe to viewing math as science.

> Why don't serious math journals require programmatically checked proofs for all publications

Presumably it has something to do with why formalized mathematics is rare: it's hard. Even relatively simple concepts become quite hard to deal with when fully formalized. I suspect that mathematics on this level is simply not amenable to the treatment that you envision (at least today).