It's because whoever made the prints is incompetent and/or didn't care. If you want good prints, you either need to do it yourself or go to a top-tier professional lab - the Local Shop* no longer cuts it. The minilabs can produce excellent prints, but they require a skilled operator to do that whereas your average corner store uses a (probably just toilet-)trained monkey.

It's also possible you made some error like underexposure that would make it very difficult to get good prints from your negatives.

* for Local People