Because they're an emerging and unproven technology? Why don't people rewrite everything in Rust? That would surely be a lot more straightforward than adopting the sorts of "formal methods" where you only write a handful of (C-language equivalent) LOC per day, across the software industry! So why doesn't it happen? There's your answer.
AI is 30+ years old too, and formal methods (especially things like solving SAT, or theorem proving) used to be considered AI. I'm not quite sure what your point is; just because something has a 30+ years history doesn't somehow make it a mature, usable tech.
Because they're an emerging and unproven technology? Why don't people rewrite everything in Rust? That would surely be a lot more straightforward than adopting the sorts of "formal methods" where you only write a handful of (C-language equivalent) LOC per day, across the software industry! So why doesn't it happen? There's your answer.