This is honestly because, I feel anyway, that filmmakers believe they can do better. That they can "fix" what they think are crappy , cheap suits from the OT. Just use modern materials and make it look like it did, it's not hard. Like you said, amateurs do it al the time.
Well I would have been fine with creating a nicer and more expensive costume that maintains the basic ANH look. But like others said, it seems pretty clear that they wanted something closer to his ESB and ROTJ look instead.