Because `const` is a js level keyword that has different semantics: it applies only to the reference binding being immutable, not the value as being so. `const a = "hello"` does have the shape `"hello"` as you’d expect, since strings are immutable in js and the `const` keyword makes the binding immutable as well. But objects are mutable, so TS can’t be a superset of javascript and change the semantics of that behavior. Hence `as const`.

This is different to the widening of value types within objects, which is a tradeoff made towards unsoundness with the belief that it would otherwise make interoperability with existing js too difficult, for which `as const` now serves as a way to explicitly opt into sounder behavior.