Because writing a browser in ATS or Frama-C is not possible. Or writing it in Coq and exporting it to ocaml and friends. Ada can be used but its mediocre at best and sucks at places where frama-C shines. A lot of pointer stuff that is easier to prove in ATS or frama-C is impossible in Ada [and by extension of that, rust] TLA+ can be used in mix with C++ just like how 90% of safety critical software is written but the development speed is very slow for it. That leaves Rust, which is a...mediocre language if you really care about safety. It has no idea of linear types that ATS has and cannot carry embedded proofs like Frama-C. Plus the culture regarding safety critical software in C/C++ is humongous and tbh fairly amazing. I cannot remember exactly but there are many projects targeting C++ for safety purposes, even the people behind frama-C are creating something called as frama-Clang to allow writing safety critical code in C++ backed by proofs.

Are there any other languages that im missing? If ATS has a prettier syntax, it'd be my candidate for writing a web browser in.