Because you can write `this[anything()]()` and it's impossible to analyze it. IDE false negative will not do anything bad, but treeshaker false negative will introduce a bug, so they have to be conservative.
That's not entirely true. Tree-shaking algorithms could have a “noDynamicAccess” option that errors on such use (only viable for applications, not libraries). Alternatively, the algorithm could be integrated with the TypeScript compiler API to allow dynamic access in some cases (e.g. where the `anything` function in your example only returns a “string literal” constant type or a union thereof, instead of the `string` type).
