Roughly, because (a) performance (b) the intention to makie it still possible to poke into an object's guts but more obvious that's what you're doing (c) if you really want the class syntax atop a classic blessed reference, p3rl.org/Object::Pad will do that if you configure the :repr (that's basically the cpan testbed for a lot of the ideas that have gone into core and written by the same LeoNerd)
