One thing that comes to mind is different TypeScript versions.
JS has ES6 and ES2020 etc and that’s enough to deal with, Typescript is younger and has been moving faster, compatibility might be difficult around the edges.
Longer term I agree with you though, I’d much rather see TS source than JS plus types when I’m looking at the source code of a module.
I don’t know why you’re being downvoted, my experience has been that minor version TS releases usually create a cascade of hundreds of type check failures even though the syntax itself is backwards-compatible.
It would be impossible for a project pulling TS sources directly to achieve stability (or even compile all dependencies with a single version of TypeScript in the first place).
In Deno with JSR, only the public API gets used for type checking because publishing enforces that the public API can be determined without type inference. So it's similar to declaration files (d.ts files) and you wouldn't see errors that occur at the non-declaration level unless someone explicitly opted out of those publishing checks (which is heavily discouraged).
JS has ES6 and ES2020 etc and that’s enough to deal with, Typescript is younger and has been moving faster, compatibility might be difficult around the edges.
Longer term I agree with you though, I’d much rather see TS source than JS plus types when I’m looking at the source code of a module.