It costs time, distracts some devs, and adds complexity for negligible safety improvement. Especially if/when the types end up being used everywhere because managers like that metric. I get using types if you have no tests, but you really need tests either way. I've done the opposite migration before, TS to JS.

Oh I forgot to qualify that I'm only talking about high level code, not things that you'd use C or Rust for. But part of the reason those langs have static types is they need to know sizes on stack at compile time.