[int_19h]

These are two different use cases. In the first case, which method the call actually dispatches to depends on obj, so it kinda makes sense for it to be syntactically distinct from other args. The pipeline syntax is normally used with free-standing functions that are statically dispatched.

Now, yes, there are languages with multimethods, where the distinction is less obvious. For those that always dispatch on all arguments, a uniform syntax makes more sense. CLOS is a good example.