It is because C# uses nominal types (C-like) and typescript uses structural types (ocalm-like).

https://en.wikipedia.org/wiki/Nominal_type_system

https://en.wikipedia.org/wiki/Structural_type_system

Although nominal types doesn't necessarily mean OOP-ish (inheritance-heavy) it is a pre-requisite (for inheritance-heavy code).

The distinction between the two is not a black/white thing but (modern) typescript (and Flow as well) is heavily focused on structural typing while C# is heavily focused on nominal typing. In fact the whole composition vs inheritance discussion fundamentally is about making types that behave in a more structural manner.